fix: change users.id and FK columns from uuid to text for Better-Auth compatibility

Better-Auth generates nanoid-style text IDs (e.g. pGud2ln2WAFHC0KYjBVKR4Rc7mM8OcTI),
but the users table used PostgreSQL uuid type, causing registration failures:
  ERROR: invalid input syntax for type uuid: "pGud2ln2WAFHC0KYjBVKR4Rc7mM8OcTI"

Changes:
- User.id: removed UUIDPrimaryKeyMixin, use explicit text PK
- UserStoreAccount.user_id: Mapped[uuid.UUID] -> Mapped[str]
- Purchase.user_id: Mapped[uuid.UUID] -> Mapped[str]
- UserResponse schema: id field from UUID -> str
- New Alembic migration 004_fix_user_id_text: drops FKs, alters column
  types, re-adds FKs (using id::text cast)

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

alembic/versions/004_fix_user_id_text.py

@@ -0,0 +1,122 @@
+"""Fix users.id UUID->text type mismatch for Better-Auth compatibility.
+
+Better-Auth generates nanoid-style text IDs (e.g. pGud2ln2WAFHC0KYjBVKR4Rc7mM8OcTI),
+but the users table was using PostgreSQL uuid type. When Better-Auth tries to INSERT
+a new user, Postgres throws:
+  ERROR: invalid input syntax for type uuid: "pGud2ln2WAFHC0KYjBVKR4Rc7mM8OcTI"
+
+The sessions, accounts, and verifications tables already use text IDs — only users,
+user_store_accounts.user_id, and purchases.user_id needed fixing.
+
+Revision ID: 004_fix_user_id_text
+Revises: 003_make_users_hashed_password_nullable
+Create Date: 2026-03-31
+"""
+
+import sqlalchemy as sa
+from sqlalchemy import text
+
+from alembic import op
+
+revision = "004_fix_user_id_text"
+down_revision = "003_make_users_hashed_password_nullable"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Step 1: Drop existing FK constraints
+    op.execute(text("ALTER TABLE user_store_accounts DROP CONSTRAINT IF EXISTS user_store_accounts_user_id_fkey"))
+    op.execute(text("ALTER TABLE purchases DROP CONSTRAINT IF EXISTS purchases_user_id_fkey"))
+
+    # Step 2: Alter users.id from uuid to text
+    op.alter_column(
+        "users",
+        "id",
+        type_=sa.Text(),
+        existing_type=sa.UUID(),
+        postgresql_using="id::text",
+    )
+
+    # Step 3: Alter user_store_accounts.user_id from uuid to text
+    op.alter_column(
+        "user_store_accounts",
+        "user_id",
+        type_=sa.Text(),
+        existing_type=sa.UUID(),
+        postgresql_using="user_id::text",
+    )
+
+    # Step 4: Alter purchases.user_id from uuid to text
+    op.alter_column(
+        "purchases",
+        "user_id",
+        type_=sa.Text(),
+        existing_type=sa.UUID(),
+        postgresql_using="user_id::text",
+    )
+
+    # Step 5: Re-add FK constraints
+    op.execute(
+        text(
+            "ALTER TABLE user_store_accounts "
+            "ADD CONSTRAINT user_store_accounts_user_id_fkey "
+            "FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE"
+        )
+    )
+    op.execute(
+        text(
+            "ALTER TABLE purchases "
+            "ADD CONSTRAINT purchases_user_id_fkey "
+            "FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE"
+        )
+    )
+
+
+def downgrade() -> None:
+    # Drop FK constraints
+    op.execute(text("ALTER TABLE user_store_accounts DROP CONSTRAINT IF EXISTS user_store_accounts_user_id_fkey"))
+    op.execute(text("ALTER TABLE purchases DROP CONSTRAINT IF EXISTS purchases_user_id_fkey"))
+
+    # Revert users.id from text to uuid
+    op.alter_column(
+        "users",
+        "id",
+        type_=sa.UUID(),
+        existing_type=sa.Text(),
+        postgresql_using="id::uuid",
+    )
+
+    # Revert user_store_accounts.user_id from text to uuid
+    op.alter_column(
+        "user_store_accounts",
+        "user_id",
+        type_=sa.UUID(),
+        existing_type=sa.Text(),
+        postgresql_using="user_id::uuid",
+    )
+
+    # Revert purchases.user_id from text to uuid
+    op.alter_column(
+        "purchases",
+        "user_id",
+        type_=sa.UUID(),
+        existing_type=sa.Text(),
+        postgresql_using="user_id::uuid",
+    )
+
+    # Re-add FK constraints (PostgreSQL will auto-name them)
+    op.execute(
+        text(
+            "ALTER TABLE user_store_accounts "
+            "ADD CONSTRAINT user_store_accounts_user_id_fkey "
+            "FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE"
+        )
+    )
+    op.execute(
+        text(
+            "ALTER TABLE purchases "
+            "ADD CONSTRAINT purchases_user_id_fkey "
+            "FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE"
+        )
+    )