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/alembic/versions/020_finalize_account_migration.py

@@ -81,8 +81,7 @@ def upgrade() -> None:
             ['account_id'], ['id'], ondelete='CASCADE'
         )
 
-    # 4. Accounts: enforce owner_id NOT NULL + FK
-    op.alter_column('accounts', 'owner_id', nullable=False)
+    # 4. Accounts: add owner FK (owner_id stays nullable due to circular FK with users)
     op.create_foreign_key(
         'fk_accounts_owner_id', 'accounts', 'users',
         ['owner_id'], ['id'], ondelete='RESTRICT'

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

backend/app/models/account.py

@@ -22,7 +22,7 @@ class Account(Base):
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     name: Mapped[str] = mapped_column(String(255), nullable=False)
     display_code: Mapped[str] = mapped_column(String(8), unique=True, nullable=False)
-    owner_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="RESTRICT"), nullable=False)
+    owner_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="RESTRICT"), nullable=True)
     stripe_customer_id: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))