fix: make api_keys.user_id index explicit in model

The index was created by the original migration but wasn't marked
in the model definition. This prevents confusion when running
alembic autogenerate.

Author: seapagan

app/models/api_key.py

@@ -18,7 +18,7 @@ class ApiKey(Base):
     id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
     )
-    user_id: Mapped[int] = mapped_column(ForeignKey("users.id"))
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id"), index=True)
     key: Mapped[str] = mapped_column(String(255), unique=True, index=True)
     name: Mapped[str] = mapped_column(String(50))
     created_at: Mapped[datetime] = mapped_column(