feat: durability and idempotency keys (#2450)

* feat: durability and idempotency keys

* feat: more durable frontend

* fix: tests

* fix: mini issues

* fix: better json validation

* fix: tests

tests/api/user/test_conversations.py

@@ -419,6 +419,94 @@ class TestGetSingleConversationHappy:
         assert response.status_code == 400
 
 
+@pytest.mark.unit
+class TestGetMessageTail:
+    """Tail-poll endpoint (``GET /api/messages/<id>/tail``) used by the
+    frontend to recover a placeholder/streaming row after a refresh.
+    """
+
+    def _seed_in_flight_message(self, pg_conn, owner_user_id):
+        from application.storage.db.repositories.conversations import (
+            ConversationsRepository,
+        )
+
+        conv_id = _seed_conversation(pg_conn, owner_user_id, name="streaming chat")
+        repo = ConversationsRepository(pg_conn)
+        msg = repo.reserve_message(
+            conv_id,
+            prompt="what's happening?",
+            placeholder_response=(
+                "Response was terminated prior to completion, try regenerating."
+            ),
+            request_id=str(uuid.uuid4()),
+            status="streaming",
+        )
+        return conv_id, str(msg["id"])
+
+    def test_owner_can_tail(self, app, pg_conn):
+        from application.api.user.conversations.routes import GetMessageTail
+
+        owner = "user-owner"
+        _, msg_id = self._seed_in_flight_message(pg_conn, owner)
+
+        with _patch_conversations_db(pg_conn), app.test_request_context(
+            f"/api/messages/{msg_id}/tail"
+        ):
+            from flask import request
+
+            request.decoded_token = {"sub": owner}
+            response = GetMessageTail().get(msg_id)
+
+        assert response.status_code == 200
+        assert response.json["status"] == "streaming"
+        assert response.json["message_id"] == msg_id
+
+    def test_shared_user_can_tail(self, app, pg_conn):
+        """A user in ``conversations.shared_with`` must be able to tail
+        an in-flight placeholder. Without the shared-with predicate
+        here, ``get_single_conversation`` lets them load the row but
+        the tail-poll silently 404s and the in-flight bubble never
+        resolves on the shared user's side.
+        """
+        from application.api.user.conversations.routes import GetMessageTail
+        from application.storage.db.repositories.conversations import (
+            ConversationsRepository,
+        )
+
+        owner = "user-owner-shared"
+        shared_user = "user-shared"
+        conv_id, msg_id = self._seed_in_flight_message(pg_conn, owner)
+        ConversationsRepository(pg_conn).add_shared_user(conv_id, shared_user)
+
+        with _patch_conversations_db(pg_conn), app.test_request_context(
+            f"/api/messages/{msg_id}/tail"
+        ):
+            from flask import request
+
+            request.decoded_token = {"sub": shared_user}
+            response = GetMessageTail().get(msg_id)
+
+        assert response.status_code == 200
+        assert response.json["message_id"] == msg_id
+
+    def test_non_member_gets_404(self, app, pg_conn):
+        from application.api.user.conversations.routes import GetMessageTail
+
+        owner = "user-owner-private"
+        intruder = "user-intruder"
+        _, msg_id = self._seed_in_flight_message(pg_conn, owner)
+
+        with _patch_conversations_db(pg_conn), app.test_request_context(
+            f"/api/messages/{msg_id}/tail"
+        ):
+            from flask import request
+
+            request.decoded_token = {"sub": intruder}
+            response = GetMessageTail().get(msg_id)
+
+        assert response.status_code == 404
+
+
 class TestUpdateConversationNameHappy:
     def test_returns_401_unauthenticated(self, app):
         from application.api.user.conversations.routes import (