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

app/rag/sources_meta.py

@@ -0,0 +1,53 @@
+"""Sources-version counter (phase 53, task 01).
+
+The single-row ``sources_meta`` table (migration 0010, the
+``kb_overview`` id=1 precedent) records **which generation of the
+knowledge base** is current. ``version`` is bumped exactly once per
+sync that actually changed the KB (task 02 — change-gated on
+``added + updated + pruned > 0``; a failed sync never bumps), so it
+doubles as the invalidation marker for saved chats: the chats API
+stamps each saved row with the current version at save time (task 03)
+and flags rows stamped with an older generation *stale* — their
+answers predate the current index and may be Regenerated against it
+(task 05).
+
+Session convention (phase 53): :func:`bump_sources_version` only
+*flushes* — the **caller** commits. The two sync paths (the admin Sync
+button, ``scripts/import_docs``) each own their session, and the bump
+must land in the same transaction as the KB changes it records.
+"""
+from __future__ import annotations
+
+from sqlalchemy.orm import Session
+
+from app.models import SourcesMeta
+
+
+def current_sources_version(db: Session) -> int:
+    """The current KB generation (one PK read of the seeded row).
+
+    Returns ``0`` when the row is absent — defensive, never raises
+    (the migration seeds the row, so absence only happens if someone
+    deleted it out-of-band).
+    """
+    row = db.get(SourcesMeta, 1)
+    if row is None:
+        return 0
+    return row.version
+
+
+def bump_sources_version(db: Session) -> int:
+    """Advance the KB generation by one; return the new version.
+
+    Upserts the single row (creating it if it was ever deleted),
+    increments ``version`` by one, and ``flush``es — the **caller**
+    commits, because each of the two sync paths owns its session and
+    the bump must commit together with the KB changes it witnessed.
+    """
+    row = db.get(SourcesMeta, 1)
+    if row is None:
+        row = SourcesMeta(id=1, version=0)
+        db.add(row)
+    row.version += 1
+    db.flush()
+    return row.version