fix: resolve template errors, orphaned sessions, and auth redirects

- Fix exercise_id undefined error in log_form.html by using scalar
  exercise_id instead of exercise.id object reference
- Clean up orphaned WorkoutSession records when all logs are deleted
- Filter empty sessions from dashboard stats (sessions, volume, streak)
- Replace broken HTTPException auth redirect with custom exception
  handler that properly returns 302 to /login

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/main.py

@@ -17,6 +17,7 @@ from sqlmodel import SQLModel, Session
 from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
 from starlette.requests import Request as StarletteRequest
 from starlette.responses import Response
+from fastapi.responses import RedirectResponse
 
 from app.models.user import User
 from app.config import get_settings
@@ -35,7 +36,7 @@ from app.routes.schedule import router as schedule_router
 from app.services.seed_service import SeedService
 from app.services.auth_service import AuthService
 from app.services.user_service import UserService
-from app.utils.auth import SESSION_COOKIE_NAME
+from app.utils.auth import SESSION_COOKIE_NAME, NotAuthenticatedError
 
 logger = structlog.get_logger(__name__)
 
@@ -91,6 +92,11 @@ def create_app() -> FastAPI:
         version="0.1.0",
     )
 
+    # Redirect unauthenticated requests to login
+    @app.exception_handler(NotAuthenticatedError)
+    async def _not_authenticated_handler(request, exc):
+        return RedirectResponse(url="/login", status_code=302)
+
     # Mount static files
     app.mount("/static", StaticFiles(directory=str(STATIC_DIR)), name="static")