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/alembic/versions/0014_merge_heads.py

@@ -7,8 +7,6 @@ Create Date: 2026-05-22 21:50:00.000000
 """
 from typing import Sequence, Union
 
-from alembic import op
-import sqlalchemy as sa
 
 # revision identifiers, used by Alembic.
 revision: str = "0014_merge_heads"