fix(dashboard): prewarm cache and expose pick evidence

services/cache_manager.py

@@ -63,6 +63,7 @@ _DASHBOARD_DATA_CACHE = {
 _DASHBOARD_CACHE_TTL = 1800
 _BASE_DIR = Path(__file__).resolve().parents[1]
 _DASHBOARD_SHARED_CACHE_FILE = _BASE_DIR / "data" / "dashboard_full_cache.pkl"
+_DASHBOARD_STALE_CACHE_FILE = _BASE_DIR / "data" / "dashboard_full_cache_stale.pkl"
 
 
 def cleanup_sales_cache():
@@ -128,7 +129,11 @@ def clear_dashboard_cache():
         'full_timestamp': None,
     })
     try:
-        os.remove(_DASHBOARD_SHARED_CACHE_FILE)
+        if os.path.exists(_DASHBOARD_SHARED_CACHE_FILE):
+            try:
+                os.replace(_DASHBOARD_SHARED_CACHE_FILE, _DASHBOARD_STALE_CACHE_FILE)
+            except OSError:
+                os.remove(_DASHBOARD_SHARED_CACHE_FILE)
     except FileNotFoundError:
         pass
     except OSError: