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/repositories/protocols.py

@@ -16,6 +16,8 @@ from app.models.ban import BanOrigin
 from app.repositories.fail2ban_db_repo import BanIpCount, BanRecord, HistoryRecord, JailBanCount
 from app.repositories.geo_cache_repo import GeoCacheRow
 from app.repositories.import_log_repo import ImportLogRow
+from app.models.blocklist import ImportRunEntry
+
 
 
 class SessionRepository(Protocol):
@@ -140,6 +142,47 @@ class ImportLogRepository(Protocol):
         ...
 
 
+class ImportRunRepository(Protocol):
+    """Protocol for tracking blocklist import runs for idempotency."""
+
+    async def get_by_source_and_hash(
+        self,
+        db: aiosqlite.Connection,
+        source_id: int,
+        content_hash: str,
+    ) -> ImportRunEntry | None:
+        """Check if a specific import (by source and content hash) has been completed."""
+        ...
+
+    async def create_pending(
+        self,
+        db: aiosqlite.Connection,
+        source_id: int,
+        content_hash: str,
+    ) -> int:
+        """Create a pending import run entry. Returns the id."""
+        ...
+
+    async def mark_completed(
+        self,
+        db: aiosqlite.Connection,
+        run_id: int,
+        imported_count: int,
+        skipped_count: int,
+    ) -> None:
+        """Mark an import run as completed with final counts."""
+        ...
+
+    async def mark_failed(
+        self,
+        db: aiosqlite.Connection,
+        run_id: int,
+        error_message: str,
+    ) -> None:
+        """Mark an import run as failed with error details."""
+        ...
+
+
 class GeoCacheRepository(Protocol):
     async def load_all(self, db: aiosqlite.Connection) -> list[GeoCacheRow]:
         ...