style: fix all ruff and eslint errors across codebase

Backend (ruff):
- Fix 106 errors: move imports to top of file (E402)
- Remove unused imports (F401)
- Add missing imports for undefined names (F821)
- Remove unused variables (F841)
- Fix test_models.py broken RefreshToken test
- Fix test_projects_api.py missing TestClient import

Frontend (eslint):
- Remove unused imports/variables across 10 files
- Fix explicit any types in client.ts and sessions.ts
- Clean up empty block statements in terminal.tsx

Quality gates: ruff (pass), eslint (pass), tsc --noEmit (pass),
pytest (98 passed, 4 pre-existing failures)

apps/api/src/services/docker_build.py

@@ -18,7 +18,6 @@ def build_image(instance_dir: str, dockerfile: str, tag: str, build_context: dic
     Returns:
         Tuple of (returncode, stdout, stderr)
     """
-    import os
     from pathlib import Path
 
     # Write Dockerfile