TASK-028: Add exception logging to fire-and-forget asyncio.create_task()

- Create logged_task() helper in backend/app/utils/async_utils.py to wrap
  fire-and-forget coroutines with exception logging
- Ensures unhandled task exceptions are always logged to structlog instead of
  silently discarded (Python 3.11+ RuntimeWarning)
- Update ban_service.py to use logged_task() for geo_cache.lookup_batch()
  background resolution
- Add comprehensive tests for logged_task() in test_async_utils.py
- Document fire-and-forget task conventions in Backend-Development.md

The logged_task() wrapper catches any exception raised in a background task,
logs it with full traceback context and task name, and never re-raises.

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

Docs/Tasks.md

@@ -1,36 +1,3 @@
-## TASK-027 — Debug compose hardcodes a publicly known weak session secret
-
-**Severity:** Medium
-
-### Where found
-`Docker/compose.debug.yml` line ~63:
-```yaml
-BANGUI_SESSION_SECRET: "${BANGUI_SESSION_SECRET:-dev-secret-do-not-use-in-production}"
-```
-
-### Why this is needed
-The fallback value `dev-secret-do-not-use-in-production` is now publicly visible in the repository. If `compose.debug.yml` is used in any environment where `BANGUI_SESSION_SECRET` is not set (e.g., a CI environment or a staging server that uses the debug compose file), all session tokens can be forged by anyone who has seen this repository.
-
-### Goal
-Remove the insecure default. Require the secret to be set explicitly before the container starts.
-
-### What to do
-1. Change to `BANGUI_SESSION_SECRET: "${BANGUI_SESSION_SECRET:?BANGUI_SESSION_SECRET must be set — generate with: python -c 'import secrets; print(secrets.token_hex(32))'}"`.
-2. Create a `.env.example` file at the project root with placeholder values and generation instructions.
-3. Add `.env` to `.gitignore` (verify it is already there).
-
-### Possible traps and issues
-- This will break `docker compose -f Docker/compose.debug.yml up` without a `.env` file. Add a clear error message and setup instructions to the README or `Instructions.md`.
-- `docker-compose.yml` (the legacy file) already uses the `:?` pattern — follow the same approach.
-
-### Docs changes needed
-- `Instructions.md` — add first-run setup instructions for the `.env` file.
-
-### Doc references
-- [Instructions.md](Instructions.md) — developer setup
-
----
-
 ## TASK-028 — Fire-and-forget `asyncio.create_task()` silently discards exceptions
 
 **Severity:** Low