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/geo_cache_flush.py

@@ -15,14 +15,15 @@ from typing import TYPE_CHECKING
 
 import structlog
 
-from app.services.geo_cache import GeoCache
 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:
     from fastapi import FastAPI
 
     from app.config import Settings
+    from app.services.geo_cache import GeoCache
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -32,6 +33,9 @@ GEO_FLUSH_INTERVAL: int = 60
 #: Stable APScheduler job ID — ensures re-registration replaces, not duplicates.
 JOB_ID: str = "geo_cache_flush"
 
+#: Maximum seconds to allow for geo cache flush to complete.
+TASK_TIMEOUT_SECONDS: int = 60
+
 
 async def _run_flush_with_resources(geo_cache: GeoCache, settings: Settings) -> None:
     """Flush the geo cache dirty set to the application database.
@@ -40,11 +44,15 @@ async def _run_flush_with_resources(geo_cache: GeoCache, settings: Settings) ->
         geo_cache: The application's GeoCache instance.
         settings: The resolved application settings used for database access.
     """
-    async with task_db(settings) as db:
-        count = await geo_cache.flush_dirty(db)
 
-    if count > 0:
-        log.debug("geo_cache_flush_ran", flushed=count)
+    async def _do_flush() -> None:
+        async with task_db(settings) as db:
+            count = await geo_cache.flush_dirty(db)
+
+        if count > 0:
+            log.debug("geo_cache_flush_ran", flushed=count)
+
+    await run_with_timeout("geo_cache_flush", _do_flush(), TASK_TIMEOUT_SECONDS)
 
 
 async def _run_flush(app: FastAPI) -> None: