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/database.py

@@ -1,8 +1,11 @@
 import os
-from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker
+
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 from sqlalchemy.orm import DeclarativeBase
 
-DATABASE_URL = os.getenv("DATABASE_URL", "postgresql+asyncpg://postgres:postgres@localhost:5432/arbret")
+DATABASE_URL = os.getenv(
+    "DATABASE_URL", "postgresql+asyncpg://postgres:postgres@localhost:5432/arbret"
+)
 
 engine = create_async_engine(DATABASE_URL)
 async_session = async_sessionmaker(engine, expire_on_commit=False)
@@ -15,4 +18,3 @@ class Base(DeclarativeBase):
 async def get_db():
     async with async_session() as session:
         yield session
-