chore(api): stabilize SQL core and enhance searchability

- Refactor SQL CRUD to use engine.connect() context managers for thread safety
- Optimize connection pooling in lib_sql_core
- Clean up app/routers/api.py to fix duplicate definitions and OpenAPI KeyError
- Add 'default_qry_str' to searchable_fields for Event, Session, Presentation, Presenter, Badge, and Journal
- Add 'event_location_name' to searchable_fields for Event Session
- Verified 20/20 E2E success via repro_intermittent_errors.py

app/object_definitions/events_registration.py

@@ -26,7 +26,7 @@ events_registration_obj_li = {
             'professional_title', 'full_name', 'affiliations', 'email', 'phone',
             'location', 'allow_tracking', 'print_count', 'print_first_datetime',
             'print_last_datetime', 'enable', 'hide', 'priority', 'sort', 'group',
-            'notes', 'created_on', 'updated_on'
+            'notes', 'created_on', 'updated_on', 'default_qry_str'
         ],
     },
     'event_badge_template': {