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>

backend/tests/test_services/test_blocklist_service.py

@@ -14,6 +14,7 @@ from app.models.blocklist import (
     ScheduleConfig,
     ScheduleFrequency,
 )
+from app.repositories import import_log_repo
 from app.services import blocklist_service
 
 # ---------------------------------------------------------------------------
@@ -115,6 +116,27 @@ class TestSourceCRUD:
         result = await blocklist_service.delete_source(db, 9999)
         assert result is False
 
+    @patch("app.utils.ip_utils.validate_blocklist_url")
+    async def test_delete_source_with_logs_raises_error(
+        self, mock_validate: AsyncMock, db: aiosqlite.Connection
+    ) -> None:
+        """delete_source raises BlocklistSourceHasLogsError when source has import logs."""
+        mock_validate.return_value = None
+        source = await blocklist_service.create_source(db, "HasLogs", "https://haslogs.test/")
+        # Create an import log for this source
+        await import_log_repo.add_log(
+            db,
+            source_id=source.id,
+            source_url="https://haslogs.test/",
+            ips_imported=10,
+            ips_skipped=0,
+            errors=None,
+        )
+        from app.exceptions import BlocklistSourceHasLogsError
+        with pytest.raises(BlocklistSourceHasLogsError) as exc_info:
+            await blocklist_service.delete_source(db, source.id)
+        assert exc_info.value.source_id == source.id
+
 
 # ---------------------------------------------------------------------------
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