feat(chat): stream model thinking over SSE and show it in a collapsible block

app/schemas.py

@@ -45,6 +45,21 @@ class SourceRef(BaseModel):
     title: str
 
 
+class ChatThinkingEvent(BaseModel):
+    """SSE thinking event: one chunk of the model's reasoning (phase 17).
+
+    PLAN §4 extension (A15, owner permission 2026-08-23): frames of the
+    shape ``{type: "thinking", text: str}`` stream ahead of the
+    ``delta`` frames in practice (the model reasons before it answers). The
+    client renders them in a collapsible "Thinking" block; the ``done``
+    event shape is unchanged and thinking text never travels on it.
+    Sibling of :class:`ChatErrorEvent`.
+    """
+
+    type: str = "thinking"
+    text: str
+
+
 class ChatDoneEvent(BaseModel):
     """Final SSE event of a chat turn: metadata for the finished answer."""