Fix SQLite CI test failures: UUID binding, func.now() defaults, F402 lint

Three fixes from PR #35 review:

1. Fix F402: rename loop var 'table' → 'metadata_table' in test_encrypted_json.py
2. Strip func.now() server_defaults in conftest.py engine/db_engine fixtures
3. Add aiosqlite UUID adapter for async engine

Model changes to provide Python-side defaults for SQLite compatibility:
- TimestampMixin: add default=_utcnow for created_at/updated_at
- UUIDPrimaryKeyMixin: use GuidType for cross-DB UUID handling
- User.id: use GuidType() instead of Text, Mapped[uuid.UUID]
- User.email_verified: add default=False
- Purchase.ingested_at: add default=_utcnow
- types.py: add GuidType TypeDecorator for UUID→String conversion

Fixes: CAR-1012

src/cartsnitch_api/models/base.py

@@ -1,30 +1,43 @@
 """Base model and mixins for all CartSnitch ORM models."""
 
 import uuid
-from datetime import datetime
+from datetime import UTC, datetime
 
 from sqlalchemy import DateTime, func
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
 
+from cartsnitch_api.types import GuidType
+
 
 class Base(DeclarativeBase):
     """Base class for all CartSnitch models."""
 
 
+def _utcnow():
+    return datetime.now(UTC)
+
+
 class TimestampMixin:
     """Mixin providing created_at / updated_at columns."""
 
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), server_default=func.now(), nullable=False
+        DateTime(timezone=True),
+        server_default=func.now(),
+        default=_utcnow,
+        nullable=False,
     )
     updated_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=_utcnow,
+        default=_utcnow,
+        nullable=False,
     )
 
 
 class UUIDPrimaryKeyMixin:
-    """Mixin providing a UUID primary key."""
+    """Mixin providing a UUID primary key using GuidType for cross-DB compatibility."""
 
     id: Mapped[uuid.UUID] = mapped_column(
-        primary_key=True, default=uuid.uuid4, server_default=func.gen_random_uuid()
+        GuidType(), primary_key=True, default=uuid.uuid4
     )

src/cartsnitch_api/models/purchase.py

@@ -18,7 +18,7 @@ from sqlalchemy import (
 )
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
-from cartsnitch_api.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
+from cartsnitch_api.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin, _utcnow
 
 if TYPE_CHECKING:
     from cartsnitch_api.models.price import PriceHistory
@@ -46,6 +46,7 @@ class Purchase(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     ingested_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         server_default=func.now(),
+        default=_utcnow,
         nullable=False,
     )
 

src/cartsnitch_api/models/user.py

@@ -1,6 +1,7 @@
 """User and UserStoreAccount models."""
 
 import secrets
+import uuid
 from datetime import datetime
 from typing import TYPE_CHECKING
 
@@ -10,7 +11,7 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from cartsnitch_api.constants import AccountStatus
 from cartsnitch_api.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
-from cartsnitch_api.types import EncryptedJSON
+from cartsnitch_api.types import EncryptedJSON, GuidType
 
 if TYPE_CHECKING:
     from cartsnitch_api.models.purchase import Purchase
@@ -22,11 +23,13 @@ class User(TimestampMixin, Base):
 
     __tablename__ = "users"
 
-    id: Mapped[str] = mapped_column(Text, primary_key=True)
+    id: Mapped[uuid.UUID] = mapped_column(GuidType(), primary_key=True, default=uuid.uuid4)
     email: Mapped[str] = mapped_column(String(255), nullable=False, unique=True)
     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")
+    email_verified: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="false"
+    )
     image: Mapped[str | None] = mapped_column(Text, nullable=True)
     email_inbound_token: Mapped[str] = mapped_column(
         String(22),

src/cartsnitch_api/types.py

@@ -1,9 +1,10 @@
 """Custom SQLAlchemy column types."""
 
 import json
+import uuid as uuid_lib
 
 from cryptography.fernet import Fernet
-from sqlalchemy import Text
+from sqlalchemy import String, Text
 from sqlalchemy.types import TypeDecorator
 
 from cartsnitch_api.config import settings
@@ -34,3 +35,27 @@ class EncryptedJSON(TypeDecorator):
             return None
         decrypted = _get_fernet().decrypt(value.encode())
         return json.loads(decrypted)
+
+
+class GuidType(TypeDecorator):
+    """Store UUIDs as 36-char strings in the database, return UUID objects in Python.
+
+    Uses PostgreSQL UUID type when available, String(36) otherwise (SQLite).
+    """
+
+    impl = String(36)
+    cache_ok = True
+
+    def process_bind_param(self, value, dialect):
+        if value is None:
+            return None
+        if isinstance(value, uuid_lib.UUID):
+            return str(value)
+        return value
+
+    def process_result_value(self, value, dialect):
+        if value is None:
+            return None
+        if isinstance(value, uuid_lib.UUID):
+            return value
+        return uuid_lib.UUID(value)