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/history_sync.py

@@ -13,6 +13,7 @@ import structlog
 
 from app.services import history_service
 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
 
 if TYPE_CHECKING:
@@ -31,16 +32,22 @@ HISTORY_SYNC_INTERVAL: int = 300
 #: Backfill window when archive is empty (seconds).
 BACKFILL_WINDOW: int = 648000
 
+#: Maximum seconds to allow for history sync to complete.
+TASK_TIMEOUT_SECONDS: int = 60
+
 
 async def _run_sync_with_settings(settings: Settings) -> None:
     socket_path: str = settings.fail2ban_socket
 
-    try:
-        async with task_db(settings) as db:
-            synced = await history_service.sync_from_fail2ban_db(db, socket_path)
-        log.info("history_sync_complete", synced=synced)
-    except Exception:
-        log.exception("history_sync_failed")
+    async def _do_sync() -> None:
+        try:
+            async with task_db(settings) as db:
+                synced = await history_service.sync_from_fail2ban_db(db, socket_path)
+            log.info("history_sync_complete", synced=synced)
+        except Exception:
+            log.exception("history_sync_failed")
+
+    await run_with_timeout("history_sync", _do_sync(), TASK_TIMEOUT_SECONDS)
 
 
 async def _run_sync(app: FastAPI) -> None: