fix: resolve circular FK between users and accounts on registration

Account.owner_id and User.account_id are both NOT NULL, creating a
circular dependency that prevents inserting either row first. Fix by:
1. Making owner_id nullable (set immediately after user creation)
2. Creating Account before User, then setting owner_id after flush
3. Removing NOT NULL enforcement on owner_id in migration 020

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

backend/app/api/endpoints/auth.py

@@ -134,35 +134,47 @@ async def register(
             detail="Email already registered"
         )
 
-    # Create new user
-    new_user = User(
-        email=user_data.email,
-        password_hash=get_password_hash(user_data.password),
-        name=user_data.name,
-        role="engineer",
-        invite_code_id=invite_code_record.id if invite_code_record else None
-    )
-    db.add(new_user)
-    await db.flush()  # Get user ID before creating account
-
     if account_invite_record:
         # Join existing account via account invite
-        new_user.account_id = account_invite_record.account_id
-        new_user.account_role = account_invite_record.role
+        new_user = User(
+            email=user_data.email,
+            password_hash=get_password_hash(user_data.password),
+            name=user_data.name,
+            role="engineer",
+            invite_code_id=invite_code_record.id if invite_code_record else None,
+            account_id=account_invite_record.account_id,
+            account_role=account_invite_record.role,
+        )
+        db.add(new_user)
+        await db.flush()
 
         # Mark account invite as used
         account_invite_record.accepted_by_id = new_user.id
         account_invite_record.used_at = datetime.now(timezone.utc)
     else:
-        # Create personal Account + free Subscription
+        # Create personal Account first (user needs account_id for NOT NULL constraint)
         new_account = Account(
             name=f"{user_data.name}'s Account",
             display_code=_generate_display_code(),
-            owner_id=new_user.id,
         )
         db.add(new_account)
         await db.flush()  # Get account ID
 
+        new_user = User(
+            email=user_data.email,
+            password_hash=get_password_hash(user_data.password),
+            name=user_data.name,
+            role="engineer",
+            invite_code_id=invite_code_record.id if invite_code_record else None,
+            account_id=new_account.id,
+            account_role="owner",
+        )
+        db.add(new_user)
+        await db.flush()  # Get user ID
+
+        # Now set account owner and create subscription
+        new_account.owner_id = new_user.id
+
         new_subscription = Subscription(
             account_id=new_account.id,
             plan="free",
@@ -170,9 +182,6 @@ async def register(
         )
         db.add(new_subscription)
 
-        new_user.account_id = new_account.id
-        new_user.account_role = "owner"
-
     # Mark platform invite code as used
     if invite_code_record:
         invite_code_record.used_by_id = new_user.id