feat(rate-limiting): add per-bucket limits and startup validation

- Add per-bucket rate limit config (ban, unban, import, config, jail, filter, action)
- Add process-local warning at startup for multi-worker deployments
- Document Redis migration path for shared state across workers
- Remove Issue #42 from Tasks.md (resolved)

Docs/Deployment.md

@@ -113,6 +113,56 @@ If fail2ban goes offline but the backend always returns 200, Docker treats the c
 ---
 
 
+## Rate Limiting
+
+Rate limiting is enforced at two levels:
+
+1. **Global middleware** — Per-IP request rate limit across all endpoints (default: 200 requests/minute per IP)
+2. **Per-bucket limits** — Stricter limits on specific operations:
+
+   | Bucket | Limit | Window | Purpose |
+   |--------|-------|--------|---------|
+   | `bans:ban` | 100/min | 60s | Ban operations |
+   | `bans:unban` | 100/min | 60s | Unban operations |
+   | `blocklist:import` | 10/hour | 3600s | Import operations |
+   | `config:update` | 50/min | 60s | Config write operations |
+   | `jail:*` | 100/min | 60s | Jail management |
+   | `filter:*` | 50/min | 60s | Filter management |
+   | `action:*` | 50/min | 60s | Action management |
+
+### Process-Local Scope
+
+**Current implementation is process-local.** Each worker maintains independent in-memory counters. In a multi-worker deployment (N workers), an attacker can send up to N × limit requests before any single worker triggers a block — effectively multiplying the allowed request rate by the number of workers.
+
+**Short-term mitigation:** The scheduler lock enforces single-worker mode. The startup warning log (`rate_limiting_process_local_only`) documents this constraint. Deploy with one worker.
+
+**Long-term solution:** Replace the in-process GlobalRateLimiter with a Redis-backed adapter. The `check_allowed()` and `check_allowed_for_bucket()` interfaces are designed for a drop-in replacement using atomic `INCR` + `EXPIRE` semantics — no changes needed in middleware or router code.
+
+### Redis Migration (Future)
+
+When migrating to Redis, replace the in-memory deque store with:
+
+```python
+# Atomic increment with expiry (pseudo-code)
+count = redis.incr(f"rl:{ip}")
+if count == 1:  # First request, set expiry
+    redis.expire(f"rl:{ip}", window_seconds)
+if count > max_requests:
+    return False, window_seconds - redis.ttl(f"rl:{ip}")
+return True, 0
+```
+
+The bucket variants use `INCR` + `EXPIRE` on `rl:{bucket}:{ip}` keys. This preserves the sliding-window semantics while providing shared state across all workers.
+
+### Monitoring
+
+Check logs for these events:
+- `global_rate_limit_exceeded` — Global middleware blocked a request (WARNING)
+- `rate_limiting_process_local_only` — Startup warning about multi-worker limitation (WARNING)
+- `rate_limiter_cleanup` — Periodic cleanup of expired entries (DEBUG)
+
+---
+
 ## CORS Configuration
 
 Cross-Origin Resource Sharing (CORS) must be explicitly configured when the frontend and backend are served from different origins.

Docs/Tasks.md

@@ -1,88 +1,3 @@
-
-### Issue #42: CRITICAL - Single-Worker Constraint Not Enforced at Startup
-
-**Where found**:
-- `backend/app/main.py` – `create_app()` factory has no worker-count validation
-- `backend/app/utils/runtime_state.py` – documents single-process requirement but never asserts it
-
-**Why this is needed**:
-In-memory structures (session cache, RuntimeState, rate-limit windows) are process-local. Running more than one Uvicorn worker silently causes each worker to diverge on shared state, leading to stale rate limits, ghost sessions, and inconsistent server status.
-
-**Goal**:
-Fail loudly at startup when a multi-worker configuration is detected, preventing silent data corruption.
-
-**What to do**:
-1. On app startup, detect `WEB_CONCURRENCY` / `--workers` > 1 and raise a `RuntimeError` with a clear message.
-2. Add an explicit assertion in `create_app()` guarded by the config value.
-3. Document the single-worker requirement prominently in `Docs/Deployment.md`.
-
-**Possible traps and issues**:
-- Gunicorn passes worker count via env; Uvicorn may not set it — check both.
-- Testing frameworks may fork workers; ensure the check is skipped in test mode.
-
-**Docs changes needed**:
-- `Docs/Deployment.md`: add "Single-Worker Requirement" section with rationale.
-
-**Doc references**:
-- `backend/app/utils/runtime_state.py` top-of-file comment
-
----
-
-### Issue #43: CRITICAL - Rate Limiting Is Process-Local
-
-**Where found**:
-- `backend/app/middleware/rate_limit.py:35-107` – global rate limiter uses an in-process sliding window
-- `backend/app/routers/bans.py:42-97` – per-endpoint rate limiting also process-local
-
-**Why this is needed**:
-With N workers an attacker can send up to N × limit requests before any single worker triggers the limit, effectively multiplying the allowed request rate.
-
-**Goal**:
-Either enforce single-worker (Issue #42) as a prerequisite and document the limitation, or replace the in-process store with a shared backend (e.g., Redis).
-
-**What to do**:
-1. Short-term: Block multi-worker deployments (Issue #42); add a warning log on startup stating rate limiting is process-local.
-2. Long-term: Abstract the rate-limit store behind an interface so a Redis adapter can be swapped in without touching middleware logic.
-
-**Possible traps and issues**:
-- Introducing Redis adds an operational dependency; consider making it optional with a feature flag.
-- Shared counters need atomic increment semantics (use `INCR` + `EXPIRE` in Redis, not GET+SET).
-
-**Docs changes needed**:
-- `Docs/Deployment.md`: document rate-limiting scope and its dependency on single-worker mode.
-
-**Doc references**:
-- `backend/app/middleware/rate_limit.py` module docstring
-
----
-
-### Issue #44: CRITICAL - Session Cache Not Invalidated Across Workers on Logout
-
-**Where found**:
-- `backend/app/dependencies.py:100-115` – cache is populated per process, never broadcast to siblings
-
-**Why this is needed**:
-After logout the revoked session token lives in other workers' caches until TTL expires. Any request routed to a worker that still has the token cached will be accepted.
-
-**Goal**:
-Ensure session revocation is immediately visible to all processes handling requests.
-
-**What to do**:
-1. Short-term: Enforce single-worker (Issue #42).
-2. Long-term: Store session cache in a shared layer (Redis / database) and invalidate atomically on logout.
-
-**Possible traps and issues**:
-- Cache reads must remain fast; a synchronous DB lookup on every request defeats the purpose.
-- Consider a hybrid: cache positive results for a short TTL, never cache negative results.
-
-**Docs changes needed**:
-- `Docs/Deployment.md`: document session cache behavior and invalidation guarantees.
-
-**Doc references**:
-- `backend/app/config.py` – `session_cache_enabled` field description
-
----
-
 ### Issue #45: HIGH - Session Cache Not Invalidated on Login
 
 **Where found**:

backend/app/middleware/rate_limit.py

@@ -8,8 +8,18 @@ Rate limits can be customized per endpoint or use a global default.
 IP addresses are extracted using the same trusted-proxy-aware logic as
 authentication to ensure consistent behavior across all rate limiting.
 
-Process-local implementation — designed for single-worker deployments where
+**Process-local implementation** — Each worker process maintains its own
-the blast radius of rate-limit bypasses is isolated to one worker.
+independent counter store. In multi-worker deployments (N workers), an
+attacker can send up to N × limit requests before any single worker triggers
+the limit. This is a fundamental limitation of in-process stores.
+
+**Short-term mitigation:** Deploy with a single worker (enforced by the
+scheduler lock). The startup warning log documents this constraint.
+
+**Long-term solution:** Replace the in-process GlobalRateLimiter with a
+Redis-backed adapter that uses atomic INCR + EXPIRE semantics. The
+check_allowed() and check_allowed_for_bucket() interfaces are designed
+to make this swap-in without touching middleware or router code.
 """
 
 from __future__ import annotations

backend/app/startup.py

@@ -302,6 +302,16 @@ async def _stage_check_worker_mode_and_acquire_lock(startup_db: Any) -> None:
             "See Docs/Architekture.md § Deployment Constraints for details."
         )
 
+    log.warning(
+        "rate_limiting_process_local_only",
+        message=(
+            "Rate limiting is process-local. With multiple workers, each worker enforces "
+            "its own independent limit — an attacker can send N × limit requests before "
+            "any worker triggers a block. Deploy with a single worker, or replace the "
+            "in-process store with a shared backend (e.g., Redis) for multi-worker setups."
+        ),
+    )
+
 
 async def _stage_init_database(app: FastAPI, settings: Settings) -> Any:
     """Initialize database schema and load setup state.

backend/app/utils/rate_limiter.py

@@ -219,9 +219,16 @@ class GlobalRateLimiter:
     request counting: when an IP exceeds the limit, the next request is blocked
     until the oldest request in the window expires.
 
-    Process-local implementation — each worker maintains independent counters.
+    **Process-local implementation** — Each worker maintains independent counters.
-    Designed for single-worker deployments where the blast radius is isolated
+    In multi-worker deployments (N workers), an attacker can send up to N × limit
-    to one worker.
+    requests before any single worker triggers a block. The single-worker scheduler
+    lock provides partial protection, but deployments requiring horizontal scaling
+    should replace this with a Redis-backed store using atomic INCR + EXPIRE.
+
+    **Long-term migration path:** The check_allowed() and check_allowed_for_bucket()
+    interfaces map directly to Redis INCR + EXPIRE. A drop-in RedisRateLimiter
+    adapter would only need to replace the deque-based in-memory store with Redis
+    calls, without touching any caller code.
 
     **How It Works:**