feat(summary): revision-gated cache with stale-while-revalidate (STATE-WP-0066)

Replace the fixed 15s TTL on GET /state/summary with per-table revision
watermarks, stale-while-revalidate background refresh, and a progress-tail
section split. SQLAlchemy write hooks invalidate core or progress sections
on mutation. Adds tests, benchmark script, and operator docs.

tests/conftest.py

@@ -52,10 +52,10 @@ def _truncate(_schema):
     from api.models import Base
     import api.routers.state as _state_router
     import api.routers.workstreams as _ws_router
+    from api.services.summary_cache import reset_summary_cache_for_tests
 
-    # Reset in-process TTL caches so stale data from a previous test can't bleed through.
-    _state_router._SUMMARY_CACHE = None
-    _state_router._SUMMARY_CACHE_AT = 0.0
+    # Reset in-process caches so stale data from a previous test can't bleed through.
+    reset_summary_cache_for_tests()
     _state_router._OVERVIEW_CACHE = None
     _state_router._OVERVIEW_CACHE_AT = 0.0
     _ws_router._INDEX_CACHE = None