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/tests/helpers.py

@@ -3,8 +3,8 @@ import uuid
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from models import User, Invite, InviteStatus
 from invite_utils import generate_invite_identifier
+from models import Invite, InviteStatus, User
 
 
 def unique_email(prefix: str = "test") -> str:
@@ -15,24 +15,24 @@ def unique_email(prefix: str = "test") -> str:
 async def create_invite_for_godfather(db: AsyncSession, godfather_id: int) -> str:
     """
     Create an invite for an existing godfather user.
-    
+
     Args:
         db: Database session
         godfather_id: ID of the existing user who will be the godfather
-        
+
     Returns:
         The invite identifier.
-        
+
     Raises:
         ValueError: If the godfather user doesn't exist.
     """
     # Verify godfather exists
     result = await db.execute(select(User).where(User.id == godfather_id))
     godfather = result.scalar_one_or_none()
-    
+
     if not godfather:
         raise ValueError(f"Godfather user with ID {godfather_id} not found")
-    
+
     # Create invite
     identifier = generate_invite_identifier()
     invite = Invite(
@@ -42,7 +42,7 @@ async def create_invite_for_godfather(db: AsyncSession, godfather_id: int) -> st
     )
     db.add(invite)
     await db.commit()
-    
+
     return identifier
 
 
@@ -50,24 +50,26 @@ async def create_invite_for_godfather(db: AsyncSession, godfather_id: int) -> st
 async def create_invite_for_registration(db: AsyncSession, godfather_email: str) -> str:
     """
     Create an invite for an existing godfather user (looked up by email).
-    
+
     The godfather must already exist in the database.
-    
+
     Args:
         db: Database session
         godfather_email: Email of the existing user who will be the godfather
-        
+
     Returns:
         The invite identifier.
-        
+
     Raises:
         ValueError: If the godfather user doesn't exist.
     """
     result = await db.execute(select(User).where(User.email == godfather_email))
     godfather = result.scalar_one_or_none()
-    
+
     if not godfather:
-        raise ValueError(f"Godfather user with email '{godfather_email}' not found. "
-                        "Create the user first using create_user_with_roles().")
-    
+        raise ValueError(
+            f"Godfather user with email '{godfather_email}' not found. "
+            "Create the user first using create_user_with_roles()."
+        )
+
     return await create_invite_for_godfather(db, godfather.id)