fix: add missing timestamps to ssh_keys model

SSHKey model was missing created_at/updated_at columns which the API
response model expected. Add TimestampMixin and Alembic migration.

apps/api/src/models/ssh_key.py

@@ -5,14 +5,14 @@ from sqlalchemy import ForeignKey, String, Text
 from sqlalchemy import Uuid as UUID
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
-from src.models.base import Base, UUIDPrimaryKeyMixin
+from src.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
 
 if TYPE_CHECKING:
     from src.models.project import Project
     from src.models.user import User
 
 
-class SSHKey(UUIDPrimaryKeyMixin, Base):
+class SSHKey(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     __tablename__ = "ssh_keys"
 
     name: Mapped[str] = mapped_column(String(255))