fix: atomic upsert for import runs (Issue #12)

Replace check-then-insert race condition with INSERT ON CONFLICT.
- upsert_pending uses RETURNING id for atomic upsert
- UNIQUE(source_id, content_hash) constraint from migration 6
- blocklist_import_workflow updated to use upsert_pending
- test_import_source_success fixed for async mock patterns

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/DATABASE_MIGRATIONS.md

@@ -121,6 +121,7 @@ rm bangui.db bangui.db-wal bangui.db-shm
 | 6 | Add import_runs table for idempotent imports |
 | 7 | Add indexes to import_log |
 | 8 | Migrate import_log.timestamp TEXT→INTEGER UNIX |
+| 9 | Change import_log.source_id FK to ON DELETE RESTRICT |
 
 ## Adding New Migrations