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

app/api/sync.py

@@ -39,7 +39,16 @@ decisions):
    CLI's no-prune default is unchanged);
 5. when the import changed the KB (added + updated > 0),
    ``regenerate_overview`` refreshes the single ``kb_overview`` row
-   (phase 31 trigger, best-effort inside).
+   (phase 31 trigger, best-effort inside);
+6. when the import changed the KB (added + updated + pruned > 0 — the
+   saved-chat invalidation gate, phase 53 task 02: a pruned document
+   can invalidate a saved answer that cited it, deliberately broader
+   than step 5's overview gate), the single-row ``sources_meta``
+   version counter is bumped exactly once in a short-lived session and
+   the resulting generation lands in the status detail as
+   ``sources_version`` (an unchanged re-sync reports the current
+   generation without advancing it). A FAILED sync never bumps — the
+   run aborts in the ``failed`` state before this step.
 
 Status is in memory: a restart mid-sync loses the running state
 (accepted — the next click re-syncs idempotently).
@@ -63,6 +72,7 @@ from app.rag.git_sources import effective_sources
 from app.rag.importer import ImportSummary, import_sources
 from app.rag.llm import LLMClient, check_models
 from app.rag.overview import regenerate_overview
+from app.rag.sources_meta import bump_sources_version, current_sources_version
 from scripts.git_sync import GitSyncError, clone_or_pull
 from scripts.import_docs import repo_name
 
@@ -202,6 +212,26 @@ async def _run_sync() -> None:
         overview = False
         if summary.added + summary.updated > 0:
             overview = await regenerate_overview(llm)
+        # Phase 53 (task 02): a sync that changed the KB advances the
+        # sources version exactly once — the saved-chat invalidation
+        # marker (task 03 stamps rows against it). The gate is
+        # deliberately broader than the overview's above: a pruned
+        # document can invalidate a saved answer that cited it, so
+        # ``pruned > 0`` bumps too. The bump commits in its own short
+        # session (the ``effective_sources`` pattern above), so it
+        # lands even if the best-effort overview then fails — the index
+        # really did change. An unchanged re-sync never bumps; it
+        # reports the current generation instead, so the detail always
+        # carries the generation the KB is now at.
+        db = SessionLocal()
+        try:
+            if summary.added + summary.updated + summary.pruned > 0:
+                sources_version = bump_sources_version(db)
+                db.commit()
+            else:
+                sources_version = current_sources_version(db)
+        finally:
+            db.close()
         _status.state = "success"
         _status.finished_at = datetime.now(UTC)
         _status.detail = {
@@ -215,6 +245,7 @@ async def _run_sync() -> None:
             "summaries": summary.summaries,
             "summary_errors": summary.summary_errors,
             "overview": overview,
+            "sources_version": sources_version,
         }
         logger.info("sync: done detail=%s", _status.detail)
     except Exception as e:  # noqa: BLE001 — a background task dies in state, see above