fix(auth): make users.hashed_password nullable

Better-Auth inserts into users without a hashed_password value (passwords
are stored in the accounts table). This was causing a NOT NULL constraint
violation and 422 FAILED_TO_CREATE_USER on sign-up.

Revision ID: 003_make_users_hashed_password_nullable
Resolves: CAR-172

Co-Authored-By: Paperclip <noreply@paperclip.ing>

api/alembic/versions/003_make_users_hashed_password_nullable.py

@@ -0,0 +1,26 @@
+"""Make users.hashed_password nullable.
+
+Better-Auth inserts users without hashed_password (passwords live in the
+accounts table). This column is now purely optional.
+
+Revision ID: 003_make_users_hashed_password_nullable
+Revises: 002_better_auth_tables
+Create Date: 2026-03-30
+"""
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "003_make_users_hashed_password_nullable"
+down_revision = "002_better_auth_tables"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.alter_column("users", "hashed_password", existing_type=sa.String(255), nullable=True)
+
+
+def downgrade() -> None:
+    op.alter_column("users", "hashed_password", existing_type=sa.String(255), nullable=False)

common/src/cartsnitch_common/models/user.py

@@ -21,7 +21,7 @@ class User(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     __tablename__ = "users"
 
     email: Mapped[str] = mapped_column(String(255), nullable=False, unique=True)
-    hashed_password: Mapped[str] = mapped_column(String(255), nullable=False)
+    hashed_password: Mapped[str | None] = mapped_column(String(255), nullable=True)
     display_name: Mapped[str | None] = mapped_column(String(100))
     email_verified: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="false")
     image: Mapped[str | None] = mapped_column(Text, nullable=True)