feat: add stateful responses API

backend/db/models.py

@@ -424,6 +424,52 @@ class RealtimeApiKey(Base):
     )
 
 
+class OpenAIResponse(Base):
+    """Persisted state for one public Responses API generation."""
+
+    __tablename__ = "openai_responses"
+
+    id: Mapped[str] = mapped_column(String(40), primary_key=True)
+    api_key_id: Mapped[str] = mapped_column(
+        String(40),
+        ForeignKey("realtime_api_keys.id", ondelete="CASCADE"),
+        index=True,
+    )
+    assistant_id: Mapped[str | None] = mapped_column(
+        String(40),
+        ForeignKey("assistants.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    conversation_id: Mapped[str] = mapped_column(
+        String(40),
+        ForeignKey("conversation_sessions.id", ondelete="CASCADE"),
+        index=True,
+    )
+    previous_response_id: Mapped[str | None] = mapped_column(
+        String(40),
+        ForeignKey("openai_responses.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    status: Mapped[str] = mapped_column(String(16), index=True, default="in_progress")
+    input_items: Mapped[list] = mapped_column(JSONB, default=list)
+    output_items: Mapped[list] = mapped_column(JSONB, default=list)
+    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)
+    dynamic_variables: Mapped[dict] = mapped_column(JSONB, default=dict)
+    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
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+
 class WebhookDelivery(Base):
     """Immutable event snapshot plus its asynchronous delivery state."""