refactor(db): extract db query layer for ws sessions

[sskmlm]

Summary

• extract the websocket session sqlite query/schema layer into dedicated api/db modules
• simplify the pre-release schema policy to one canonical schema and add session project_id
• add focused tests for ws session router and db initialization behavior

Notes

• this is the first split PR from feat(ws-support): cp websocket runtime support with browser chat #475
• pre-release ws session databases with older experimental schemas should be deleted and recreated

Testing

• ruff check --fix code_puppy/api/db/connection.py code_puppy/api/db/queries.py code_puppy/api/routers/ws_sessions.py code_puppy/api/tests/test_db_connection.py tests/api/test_ws_sessions_router.py
• ruff format code_puppy/api/db/connection.py code_puppy/api/db/queries.py code_puppy/api/routers/ws_sessions.py code_puppy/api/tests/test_db_connection.py tests/api/test_ws_sessions_router.py
• pytest -q code_puppy/api/tests/test_backfill_pydantic_json.py code_puppy/api/tests/test_db_sql_loader.py code_puppy/api/tests/test_db_connection.py tests/api/test_ws_sessions_router.py