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/src/models/__init__.py

@@ -1,10 +1,9 @@
 from src.models.base import Base
 from src.models.git_repository import GitRepository
 from src.models.project import Project
-from src.models.refresh_token import RefreshToken
 from src.models.ssh_key import SSHKey
 from src.models.tool_type import ToolType
 from src.models.user import User
 from src.models.user_config import UserConfig
 
-__all__ = ["Base", "GitRepository", "Project", "RefreshToken", "SSHKey", "ToolType", "User", "UserConfig"]
+__all__ = ["Base", "GitRepository", "Project", "SSHKey", "ToolType", "User", "UserConfig"]