Make background tasks idempotent - prevent duplicate bans on retry

CRITICAL FIX: Background tasks (especially blocklist_import) crashed mid-execution,
leaving partial state. On retry, the same bans were applied again, causing duplicates.

Solution: Content-hash based operation tracking for blocklist imports:
- Added import_runs table (migration 6) to track operations by source + content hash
- Before banning, check if this exact content has already been imported
- If completed: skip banning (already done), optionally re-warm cache
- If new or failed: proceed with ban and mark as completed or failed

Changes:
- Database: Migration 6 adds import_runs table with operation state tracking
- Model: Added ImportRunEntry for import run records
- Repository: New import_run_repo module with CRUD operations
- Workflow: Updated blocklist_import_workflow to check operation history before banning
- Dependencies: Registered import_run_repo for dependency injection
- Tests: Added test_import_source_idempotent_on_retry and test_import_source_different_content_not_reused
- Documentation: Added Task Idempotency section to Backend-Development.md

Verification:
- All 7 import tests pass (5 existing + 2 new idempotency tests)
- Type checking: mypy --strict ✅
- Linting: ruff ✅
- No API changes, backwards compatible via automatic migration

Fixes: Background tasks not idempotent #CRITICAL

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

backend/app/tasks/session_cleanup.py

@@ -16,6 +16,7 @@ import structlog
 
 from app.repositories import session_repo
 from app.tasks.db import task_db
+from app.tasks.timeout_utils import run_with_timeout
 from app.utils.runtime_state import get_effective_settings
 from app.utils.time_utils import utc_now
 
@@ -32,6 +33,9 @@ SESSION_CLEANUP_INTERVAL: int = 6 * 60 * 60  # 6 hours
 #: Stable APScheduler job ID — ensures re-registration replaces, not duplicates.
 JOB_ID: str = "session_cleanup"
 
+#: Maximum seconds to allow for session cleanup to complete.
+TASK_TIMEOUT_SECONDS: int = 30
+
 
 async def _run_cleanup_with_resources(settings: Settings) -> None:
     """Delete all expired sessions from the database.
@@ -39,11 +43,15 @@ async def _run_cleanup_with_resources(settings: Settings) -> None:
     Args:
         settings: The resolved application settings used for database access.
     """
-    now_iso = utc_now().isoformat()
-    async with task_db(settings) as db:
-        deleted_count = await session_repo.delete_expired_sessions(db, now_iso)
 
-    log.info("session_cleanup_ran", deleted_count=deleted_count, cutoff_time=now_iso)
+    async def _do_cleanup() -> None:
+        now_iso = utc_now().isoformat()
+        async with task_db(settings) as db:
+            deleted_count = await session_repo.delete_expired_sessions(db, now_iso)
+
+        log.info("session_cleanup_ran", deleted_count=deleted_count, cutoff_time=now_iso)
+
+    await run_with_timeout("session_cleanup", _do_cleanup(), TASK_TIMEOUT_SECONDS)
 
 
 async def _run_cleanup(app: FastAPI) -> None: