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/scheduler_lock_heartbeat.py

@@ -16,6 +16,7 @@ from typing import TYPE_CHECKING
 import structlog
 
 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.scheduler_lock import update_scheduler_lock_heartbeat
 
@@ -32,6 +33,9 @@ SCHEDULER_LOCK_HEARTBEAT_INTERVAL: int = 10
 #: Stable APScheduler job ID — ensures re-registration replaces, not duplicates.
 JOB_ID: str = "scheduler_lock_heartbeat"
 
+#: Maximum seconds to allow for scheduler lock heartbeat to complete.
+TASK_TIMEOUT_SECONDS: int = 5
+
 
 async def _update_heartbeat_with_resources(settings: Settings) -> None:
     """Update the scheduler lock heartbeat timestamp.
@@ -43,16 +47,20 @@ async def _update_heartbeat_with_resources(settings: Settings) -> None:
     Args:
         settings: The resolved application settings used for database access.
     """
-    async with task_db(settings) as db:
-        success = await update_scheduler_lock_heartbeat(db)
 
-    if success:
-        log.debug("scheduler_lock_heartbeat_updated")
-    else:
-        log.warning(
-            "scheduler_lock_heartbeat_failed",
-            message="Failed to update heartbeat; we may have lost the lock.",
-        )
+    async def _do_update() -> None:
+        async with task_db(settings) as db:
+            success = await update_scheduler_lock_heartbeat(db)
+
+        if success:
+            log.debug("scheduler_lock_heartbeat_updated")
+        else:
+            log.warning(
+                "scheduler_lock_heartbeat_failed",
+                message="Failed to update heartbeat; we may have lost the lock.",
+            )
+
+    await run_with_timeout("scheduler_lock_heartbeat", _do_update(), TASK_TIMEOUT_SECONDS)
 
 
 async def _update_heartbeat(app: FastAPI) -> None: