[Bug]: Clean up app.yaml entry point and env vars

Parent PRD

#188

What to build

Clean up src/app.yaml to be the authoritative runtime config:

1. Remove dead env var: Delete FRONTEND_STATIC_DIR — the backend hardcodes Path(__file__).parent.parent / "static" in app.py line 103 and never reads this env var.
2. Add missing env vars: Add DATABRICKS_CATALOG and DATABRICKS_SCHEMA with sensible defaults (e.g., app_data and app_ontos). These are required by config.py lines 54-55 and were previously only set via the bundle's app_config override (which is being removed in the sibling issue).
3. Verify entry point: Confirm python backend/src/app.py works correctly as the production command (it uses the if __name__ block at line 411 which calls uvicorn.run()).

Acceptance criteria

• FRONTEND_STATIC_DIR env var removed from app.yaml
• DATABRICKS_CATALOG env var added with default value
• DATABRICKS_SCHEMA env var added with default value
• Entry point python backend/src/app.py verified working
• No references to FRONTEND_STATIC_DIR remain in any config file

Blocked by

None - can start immediately

User stories addressed

• User story 7: app.yaml as single source of truth for runtime configuration
• User story 13: Dead configuration removed

Start with src/app.yaml, then compare its environment variables with the references in config.py lines 54-55 and the hardcoded static path in app.py line 103. Run python backend/src/app.py and verify the app.yaml changes, entry point, and absence of FRONTEND_STATIC_DIR from configuration files meet the acceptance criteria.