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/app/exceptions.py

@@ -482,6 +482,22 @@ class BlocklistSourceNotFoundError(NotFoundError):
         return {"source_id": self.source_id}
 
 
+class BlocklistSourceHasLogsError(ConflictError):
+    """Raised when attempting to delete a blocklist source that has import logs."""
+
+    error_code: str = "blocklist_source_has_logs"
+
+    def __init__(self, source_id: int) -> None:
+        self.source_id = source_id
+        super().__init__(
+            f"Blocklist source {source_id} cannot be deleted because it has import logs. "
+            "Delete the import logs first."
+        )
+
+    def get_error_metadata(self) -> dict[str, str | int | float | bool | None]:
+        return {"source_id": self.source_id}
+
+
 class HistoryNotFoundError(NotFoundError):
     """Raised when no history is found for the given IP."""