feat(chat): invalidate saved chats on sources sync — versioned stamps, stale marker, Regenerate against the new index

app/schemas.py

@@ -387,7 +387,7 @@ def _drop_absent_share_url(model: BaseModel, handler: SerializerFunctionWrapHand
 
 class SavedChatOut(BaseModel):
     """One saved chat, full payload (create/get/put response, phase 50;
-    ``share_url``, phase 51 task 02).
+    ``share_url``, phase 51 task 02; ``stale``, phase 53 task 03).
 
     ``messages`` round-trips the ``bor.chat.v1`` record list losslessly
     — the restore path is pixel-identical by construction.
@@ -395,6 +395,13 @@ class SavedChatOut(BaseModel):
     ``share_url`` (phase 51): ``"/shared/<token>"`` while the chat is
     shared, ABSENT from the JSON when unshared (``None`` → dropped by
     :func:`_drop_absent_share_url` — no ``null`` in the wire shape).
+
+    ``stale`` (phase 53): true iff the row's ``sources_version`` stamp
+    is behind the current ``sources_meta`` generation — the answer
+    predates the latest KB-changing sync. Computed server-side (the
+    client never does staleness math); ``SharedChatOut`` deliberately
+    carries no staleness surface (the public snapshot is frozen by
+    design, phase 51).
     """
 
     id: uuid.UUID
@@ -404,6 +411,9 @@ class SavedChatOut(BaseModel):
     message_count: int
     messages: list[ChatMessage]
     share_url: str | None = None
+    #: Required (no default): the API must always compute staleness
+    #: server-side — there is no wire shape without the flag.
+    stale: bool
 
     @model_serializer(mode="wrap")
     def _serialize(self, handler: SerializerFunctionWrapHandler) -> Any:
@@ -418,7 +428,9 @@ class SavedChatRow(BaseModel):
     (Title, Messages count, Updated). ``share_url`` is populated here so
     the History page's Share column renders straight from ``GET
     /api/chats`` — no second fetch per row (``None`` → absent, the same
-    omission rule as :class:`SavedChatOut`).
+    omission rule as :class:`SavedChatOut`). ``stale`` (phase 53) feeds
+    the History page's Stale column the same way: one ``GET`` powers
+    every column.
     """
 
     id: uuid.UUID
@@ -426,6 +438,8 @@ class SavedChatRow(BaseModel):
     updated_at: datetime
     message_count: int
     share_url: str | None = None
+    #: Required (no default) — see :attr:`SavedChatOut.stale`.
+    stale: bool
 
     @model_serializer(mode="wrap")
     def _serialize(self, handler: SerializerFunctionWrapHandler) -> Any: