fix(db): enable PRAGMA foreign_keys = ON on every connection

SQLite defaults foreign_keys to OFF, which meant every ondelete=CASCADE
and ondelete=SET NULL declared on an FK was inert. Bulk Query.delete()
calls — which bypass ORM cascade — then silently orphaned child rows,
and Paper.journal_id would not NULL out when a Journal was deleted.

Wiring the pragma into apply_performance_pragmas (which is already
registered via event.listen(engine, "connect")) makes every pooled
connection honor DDL-level cascade.

src/local_deep_research/database/sqlcipher_utils.py

@@ -438,6 +438,11 @@ def apply_performance_pragmas(cursor_or_conn: Any) -> None:
     cursor_or_conn.execute("PRAGMA temp_store = MEMORY")
     cursor_or_conn.execute("PRAGMA busy_timeout = 10000")  # 10 second timeout
 
+    # SQLite defaults foreign_keys to OFF — without this every
+    # ondelete="CASCADE"/"SET NULL" declared on an FK is inert,
+    # including for raw-SQL DELETEs issued by Query.delete().
+    cursor_or_conn.execute("PRAGMA foreign_keys = ON")
+
     # Cache size - registry validates min/max range
     cache_mb = get_env_setting("db_config.cache_size_mb", 64)
     cache_pages = -(cache_mb * 1024)  # Negative for KB cache size

src/local_deep_research/web/routes/research_routes.py

@@ -925,7 +925,9 @@ def clear_history():
                     except Exception:
                         logger.exception("Error removing report file")
 
-            # Delete records from the database, except active research
+            # Query.delete() bypasses ORM cascade; child rows clean up
+            # via DDL-level ondelete="CASCADE" only because PRAGMA
+            # foreign_keys = ON is set on every connection.
             if active_ids:
                 db_session.query(ResearchHistory).filter(
                     ~ResearchHistory.id.in_(active_ids)