Add ProductBase, ProductPublic, ProductWithInventory and
SkinConditionSnapshotBase, SkinConditionSnapshotPublic. Table models now inherit
from their Base counterpart and override JSON fields with sa_column. All
field_serializer hacks removed; FastAPI response models use the non-table Public
classes so Pydantic coerces raw DB dicts → typed models cleanly. ProductCreate
and SnapshotCreate now simply inherit their respective Base classes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>