feat: simplify auth flow - replace JWT with session cookies

Replace complex JWT + refresh token authentication with simple
session-based auth using signed cookies.

**Removed:**
- JWT token service (jwt_service.py)
- Refresh token store (refresh_store.py)
- Refresh token model and database table
- JWKS fetching and OIDC token verification
- python-jose dependency

**Added:**
- Session service (session.py) with HMAC-SHA256 signed cookies
- Auth dependencies module for shared auth logic
- Session-based auth endpoints

**Updated:**
- All API endpoints to use session-based auth
- Config: removed JWT settings, added SESSION_SECRET/SESSION_TTL_HOURS
- Tests: rewritten for session-based flow
- Frontend: no changes needed (already uses cookies)

Quality gates: ruff ✓, mypy ✓, typecheck ✓, lint ✓

apps/api/pyproject.toml

@@ -11,7 +11,6 @@ dependencies = [
     "alembic>=1.12.0",
     "pydantic>=2.5.0",
     "pydantic-settings>=2.1.0",
-    "python-jose[cryptography]>=3.3.0",
     "python-multipart>=0.0.6",
     "httpx>=0.25.0",
     "structlog>=23.2.0",