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/lib_api_crud_v3.py

@@ -107,10 +107,11 @@ def apply_forced_account_filter(and_qry_dict: Optional[Dict], account: AccountCo
         from app import lib_sql_core
         from sqlalchemy import text
         try:
-            lib_sql_core.db.execute(text(f"SELECT `{target_col}` FROM `{table_name}` LIMIT 0"))
-        except Exception:
-            log.warning(f"Forced account filter skipped: Column '{target_col}' not found in '{table_name}'.")
-            return forced
+            with lib_sql_core.engine.connect() as conn:
+                conn.execute(text(f"SELECT `{target_col}` FROM `{table_name}` LIMIT 0"))
+            has_col = True
+        except:
+            has_col = False
         
     forced[target_col] = account.account_id
     return forced