feat(ui): explicit chat state machine — typing indicator, streaming progress, timeout and error recovery

app/schemas.py

@@ -34,6 +34,18 @@ class ChatDoneEvent(BaseModel):
     suggestions: list[str] = []
 
 
+class ChatErrorEvent(BaseModel):
+    """SSE error event: a turn that cannot complete (PLAN §4).
+
+    The client's loading-feedback state machine (phase 06) keys off this
+    exact shape — ``{type: "error", detail: str}`` — to flip to the error
+    state and re-enable the send button.
+    """
+
+    type: str = "error"
+    detail: str
+
+
 class DocSummary(BaseModel):
     """One indexed document as shown on the Sources page / API."""