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/models/blocklist.py

@@ -78,6 +78,28 @@ class ImportLogListResponse(PaginatedListResponse[ImportLogEntry]):
 
     pass
 
+
+# ---------------------------------------------------------------------------
+# Import run tracking (for idempotency)
+# ---------------------------------------------------------------------------
+
+class ImportRunEntry(BanGuiBaseModel):
+    """Tracks a unique blocklist import run by source and content hash.
+
+    Used to detect re-runs and prevent duplicate bans when the scheduler
+    retries after a crash.
+    """
+
+    id: int
+    source_id: int
+    content_hash: str
+    status: str  # 'pending' | 'completed' | 'failed'
+    imported_count: int
+    skipped_count: int
+    error_message: str | None
+    created_at: str
+    updated_at: str
+
 # ---------------------------------------------------------------------------
 # Schedule
 # ---------------------------------------------------------------------------