Document process-local auth session cache semantics

Clarify that dependencies.py session cache is process-local and not cluster-safe, and document the limitation in architecture docs.

Docs/Architekture.md

@@ -657,6 +657,7 @@ BanGUI maintains its **own SQLite database** (separate from the fail2ban databas
 - The frontend `AuthProvider` checks session validity on mount and redirects to `/login` if invalid.
 - The backend `dependencies.py` provides an `authenticated` dependency that validates the session cookie on every protected endpoint.
 - **Session validation cache** — validated session tokens are cached in memory for 10 seconds (`_session_cache` dict in `dependencies.py`) to avoid a SQLite round-trip on every request from the same browser. The cache is invalidated immediately on logout.
+  This cache is process-local and not safe for multi-worker or distributed deployments. A clustered deployment should replace `_session_cache` with a shared cache or remove it entirely.
 - **Setup-completion flag** — once `is_setup_complete()` returns `True`, the result is stored in `app.state._setup_complete_cached`. The `SetupRedirectMiddleware` skips the DB query on all subsequent requests, removing 1 SQL query per request for the common post-setup case.
 
 ---

Docs/Tasks.md

@@ -25,8 +25,8 @@ Reference: `Docs/Refactoring.md` for full analysis of each issue.
   - Fix: remove the unused `app.state.db` branch and always open/close a dedicated task-local connection, or intentionally add a shared DB connection to `app.state` and manage its lifecycle.
   - Expected outcome: background jobs have predictable DB lifecycle, avoid hidden bugs from stale connection assumptions, and task code is simpler.
 
-- `backend/app/dependencies.py` contains an in-memory process-local session cache for auth tokens. This optimization is valid for a single-process server, but it is not cluster-safe for multi-worker or distributed deployments.
-  - Fix: either document the single-process limitation clearly, or replace `_session_cache` with an external shared cache (Redis/Memcached) or eliminate it if eventual cluster support is required.
+- Status: **done** — `backend/app/dependencies.py` contains an in-memory process-local session cache for auth tokens. This optimization is valid for a single-process server, but it is not cluster-safe for multi-worker or distributed deployments.
+  - Fix: document the single-process limitation clearly in code and project docs.
   - Expected outcome: authentication behavior is consistent across deployment modes, and session invalidation works correctly in multi-worker setups.
 
 - `backend/app/main.py` uses local imports inside `_lifespan()` to avoid circular dependencies, indicating that startup logic is tightly coupled with services.

backend/app/dependencies.py

@@ -46,6 +46,10 @@ _COOKIE_NAME = "bangui_session"
 #: How long (seconds) a validated session token is served from the in-memory
 #: cache without re-querying SQLite.  Eliminates repeated DB lookups for the
 #: same token arriving in near-simultaneous parallel requests.
+#:
+#: NOTE: this cache is process-local and is not cluster-safe. In multi-worker
+#: or distributed deployments, each process maintains its own cache, so logout
+#: invalidation and revocation may be delayed unless a shared cache is used.
 _SESSION_CACHE_TTL: float = 10.0
 
 #: ``token → (Session, cache_expiry_monotonic_time)``
@@ -56,6 +60,7 @@ def clear_session_cache() -> None:
     """Flush the entire in-memory session validation cache.
 
     Useful in tests to prevent stale state from leaking between test cases.
+    This only affects the current process.
     """
     _session_cache.clear()
 
@@ -64,7 +69,9 @@ def invalidate_session_cache(token: str) -> None:
     """Evict *token* from the in-memory session cache.
 
     Must be called during logout so the revoked token is no longer served
-    from cache without a DB round-trip.
+    from cache without a DB round-trip. This invalidation is local to the
+    current process; a clustered deployment would need a shared cache for
+    global invalidation.
 
     Args:
         token: The session token to remove.