feat(feedback): prompt for a store review after successful sessions

Add expo-store-review (SDK 54-matched via `expo install`) and wire a
one-time in-app rating prompt into the SSE busy->idle "session completed"
transition in stores/events.ts — the same signal that already drives the
"Task completed" notification, so it only fires on genuine success, never
on session.error.

State (success count, one-time "asked" flag) persists in expo-secure-store,
mirroring the consent pattern in telemetry.ts. The threshold check is split
into store-review-policy.ts, free of expo imports, so it's unit-testable
with plain `node --test` (same split as buildAuth in auth.ts).

F-Droid/Play-Services-absent safety comes from the library itself:
StoreReview.isAvailableAsync() resolves false there, so requestReview() is
never called and there's no store-URL fallback configured in app.json.

package.json

@@ -30,6 +30,7 @@
     "expo-secure-store": "~15.0.8",
     "expo-speech-recognition": "3.0.1",
     "expo-status-bar": "~3.0.9",
+    "expo-store-review": "~9.0.9",
     "react": "19.1.0",
     "react-native": "0.81.5",
     "react-native-gesture-handler": "~2.28.0",
@@ -55,4 +56,4 @@
       ]
     }
   }
-}
+}