feat: harden public OpenAI API contracts

backend/db/models.py

@@ -428,6 +428,13 @@ class OpenAIResponse(Base):
     """Persisted state for one public Responses API generation."""
 
     __tablename__ = "openai_responses"
+    __table_args__ = (
+        UniqueConstraint(
+            "api_key_id",
+            "idempotency_key",
+            name="uq_openai_response_idempotency",
+        ),
+    )
 
     id: Mapped[str] = mapped_column(String(40), primary_key=True)
     api_key_id: Mapped[str] = mapped_column(
@@ -458,13 +465,21 @@ class OpenAIResponse(Base):
     context_messages: Mapped[list] = mapped_column(JSONB, default=list)
     runtime_state: Mapped[dict] = mapped_column(JSONB, default=dict)
     error: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
+    usage: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
     dynamic_variables: Mapped[dict] = mapped_column(JSONB, default=dict)
+    idempotency_key: Mapped[str | None] = mapped_column(
+        String(255), nullable=True
+    )
+    request_hash: Mapped[str | None] = mapped_column(String(64), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()
     )
     completed_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )
+    deleted_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True, index=True
+    )
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
     )