Add ruff linter/formatter for Python

- Add ruff as dev dependency
- Configure ruff in pyproject.toml with strict 88-char line limit
- Ignore B008 (FastAPI Depends pattern is standard)
- Allow longer lines in tests for readability
- Fix all lint issues in source files
- Add Makefile targets: lint-backend, format-backend, fix-backend

backend/main.py

@@ -1,19 +1,20 @@
 """FastAPI application entry point."""
+
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from database import engine, Base
-from routes import sum as sum_routes
-from routes import counter as counter_routes
+from database import Base, engine
 from routes import audit as audit_routes
-from routes import profile as profile_routes
-from routes import invites as invites_routes
 from routes import auth as auth_routes
-from routes import meta as meta_routes
 from routes import availability as availability_routes
 from routes import booking as booking_routes
+from routes import counter as counter_routes
+from routes import invites as invites_routes
+from routes import meta as meta_routes
+from routes import profile as profile_routes
+from routes import sum as sum_routes
 from validate_constants import validate_shared_constants
 
 
@@ -22,7 +23,7 @@ async def lifespan(app: FastAPI):
     """Create database tables on startup and validate constants."""
     # Validate shared constants match backend definitions
     validate_shared_constants()
-    
+
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
     yield