Add session cleanup task and update documentation

- Implement session_cleanup task for removing expired sessions
- Add comprehensive tests for session cleanup functionality
- Update architecture and task documentation
- Integrate cleanup task into application startup

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,37 +1,3 @@
-## TASK-007 — No rate limiting on the login endpoint
-
-**Severity:** High
-
-### Where found
-`backend/app/routers/auth.py` — `POST /api/auth/login`. No rate-limiting middleware, no lockout logic.
-
-### Why this is needed
-BanGUI uses a single master password for all access. An attacker with network access to the login endpoint can attempt unlimited passwords per second. With no lockout or slowdown, a 10-character password with the required complexity (upper, digit, special) can be brute-forced offline or via the API.
-
-### Goal
-Limit login attempts per IP address to make brute-force attacks infeasible.
-
-### What to do
-1. Add [`slowapi`](https://github.com/laurents/slowapi) (or a simple in-memory counter) to rate-limit `POST /api/auth/login`.
-2. Limit to 5 attempts per minute per IP. Return `429 Too Many Requests` with a `Retry-After` header on excess.
-3. For the in-memory approach: use a `dict[str, deque[float]]` keyed by IP, storing attempt timestamps, cleaned up by a background task.
-4. Consider a 10-second `asyncio.sleep` on wrong password to further slow down attacks (already somewhat mitigated by bcrypt cost factor).
-
-### Possible traps and issues
-- Behind nginx, `request.client.host` is the proxy IP. Read the real IP from `X-Forwarded-For` only when the request comes from a trusted proxy (the nginx container's IP). Do not blindly trust `X-Forwarded-For` — it can be spoofed.
-- The in-memory rate-limiter is process-local (see TASK-002/003) — in a multi-worker setup, each worker has its own counter. Single-worker constraint limits the blast radius.
-- `slowapi` integrates cleanly with FastAPI and handles the `X-Real-IP` / `X-Forwarded-For` header logic.
-
-### Docs changes needed
-- `Features.md` — document login rate limiting.
-- `Backend-Development.md` — rate limiting conventions.
-
-### Doc references
-- [Features.md](Features.md) — authentication
-- [Backend-Development.md](Backend-Development.md) — security patterns
-
----
-
 ## TASK-008 — `delete_expired_sessions` never scheduled — sessions table grows unbounded
 
 **Severity:** Medium