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

app/models.py

@@ -23,7 +23,14 @@ Data model — see ``.agent/PLAN.md`` §Data Model:
                       14 shape) — phase 50; ``/api/chat`` stays
                       stateless; ``share_token`` (NULL = private,
                       ``uuid4`` = publicly readable at
-                      ``/shared/<token>``) — phase 51.
+                      ``/shared/<token>``) — phase 51; ``sources_version``
+                      (the KB generation the conversation was saved
+                      against; 0 = the pre-counter KB, stale on the
+                      first bump) — phase 53.
+* ``sources_meta``   — single-row sources-version counter: which
+                      generation of the knowledge base is current,
+                      bumped exactly once per KB-changing sync so saved
+                      chats can be marked stale (phase 53).
 """
 from __future__ import annotations
 
@@ -146,6 +153,33 @@ class KbOverview(Base):
     )
 
 
+class SourcesMeta(Base):
+    """Single-row sources-version counter (phase 53).
+
+    Exactly one row (``id = 1``, seeded by migration 0010 — the
+    ``kb_overview`` id=1 precedent) holds the current **generation** of
+    the knowledge base. ``version`` is bumped exactly once per sync that
+    actually changed the KB (phase 53, task 02 — change-gated on
+    ``added + updated + pruned > 0``), so it doubles as the invalidation
+    marker for saved chats: a ``saved_chats`` row stamped with an older
+    generation is *stale* — its answers predate the current index and
+    may be Regenerated against it (phase 53, tasks 03/05). The row is
+    seeded by the migration (not lazily on first bump), so
+    :func:`app.rag.sources_meta.current_sources_version` is a plain PK
+    read.
+    """
+
+    __tablename__ = "sources_meta"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, server_default="1")
+    #: The KB generation. 0 = the pre-counter KB (everything indexed
+    #: before phase 53); incremented by one per KB-changing sync.
+    version: Mapped[int] = mapped_column(Integer, server_default="0")
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+
 class GitSource(Base):
     """One admin-managed source (phase 35; kind discriminator, phase 38).
 
@@ -203,6 +237,13 @@ class SavedChat(Base):
     share_token: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), unique=True, nullable=True
     )
+    #: The sources version (KB generation) the conversation was saved
+    #: against (phase 53): stamped by the API at save time. Existing
+    #: rows (saved before the counter existed) stamp ``0`` — "the
+    #: pre-counter KB" — and become stale on the first KB-changing sync
+    #: (stale = ``sources_version < current``, computed server-side by
+    #: the chats API, phase 53 task 03).
+    sources_version: Mapped[int] = mapped_column(Integer, server_default="0")
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()