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>

src/cartsnitch_api/models/purchase.py

@@ -32,7 +32,7 @@ class Purchase(UUIDPrimaryKeyMixin, TimestampMixin, Base):
 
     __tablename__ = "purchases"
 
-    user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
+    user_id: Mapped[str] = mapped_column(ForeignKey("users.id"), nullable=False)
     store_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("stores.id"), nullable=False)
     store_location_id: Mapped[uuid.UUID | None] = mapped_column(ForeignKey("store_locations.id"))
     receipt_id: Mapped[str] = mapped_column(String(200), nullable=False)

src/cartsnitch_api/models/user.py

@@ -4,7 +4,7 @@ import uuid
 from datetime import datetime
 from typing import TYPE_CHECKING
 
-from sqlalchemy import DateTime, ForeignKey, String, UniqueConstraint
+from sqlalchemy import DateTime, ForeignKey, String, Text, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from cartsnitch_api.constants import AccountStatus
@@ -16,11 +16,12 @@ if TYPE_CHECKING:
     from cartsnitch_api.models.store import Store
 
 
-class User(UUIDPrimaryKeyMixin, TimestampMixin, Base):
+class User(TimestampMixin, Base):
     """Application user."""
 
     __tablename__ = "users"
 
+    id: Mapped[str] = mapped_column(Text, primary_key=True)
     email: Mapped[str] = mapped_column(String(255), nullable=False, unique=True)
     hashed_password: Mapped[str] = mapped_column(String(255), nullable=False)
     display_name: Mapped[str | None] = mapped_column(String(100))
@@ -36,7 +37,7 @@ class UserStoreAccount(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     __tablename__ = "user_store_accounts"
     __table_args__ = (UniqueConstraint("user_id", "store_id", name="uq_user_store_account"),)
 
-    user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
+    user_id: Mapped[str] = mapped_column(ForeignKey("users.id"), nullable=False)
     store_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("stores.id"), nullable=False)
     session_data: Mapped[dict | None] = mapped_column(EncryptedJSON)
     session_expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))

src/cartsnitch_api/schemas.py

@@ -16,7 +16,7 @@ class UpdateUserRequest(BaseModel):
 
 
 class UserResponse(BaseModel):
-    id: UUID
+    id: str
     email: str
     display_name: str
     created_at: datetime