chore(pwa): tighten audio cache eviction (30d → 7d, purgeOnQuotaError)

Workbox's CacheFirst rule for /audio/*.mp3 used a 30-day age-based
TTL. Approximate LRU by dropping to 7d — a voice clip stays cached as
long as it's played at least once a week, otherwise it falls out and
re-fetches on next play. Each clip is <200KB and same-origin so the
re-fetch cost is negligible. Also flip purgeOnQuotaError so the
runtime cache yields first under storage pressure.

Default voice precache (vite.config.js additionalManifestEntries)
unaffected — it's a separate workbox flow with its own revision.

plans/260428-0927-implement-todo-backlog/phase-08-audio-cache-lru.md

@@ -1,9 +1,10 @@
 ---
 name: Audio cache LRU rule
 phase: 8
-status: todo
+status: completed
 priority: low
 effort: 30m
+completed: 2026-04-28
 ---
 
 # Phase 8 — Audio cache LRU rule

plans/260428-0927-implement-todo-backlog/plan.md

@@ -24,7 +24,7 @@ YAGNI — parking-lot features and upstream-blocked items skipped.
 | 5 | Per-row "Chờ" indicator ✅ | `phase-05-cho-row-indicator.md` |
 | 6 | Confetti polish (threshold + variety) ✅ | `phase-06-confetti-polish.md` |
 | 7 | Strict CSP via hashed inline script ✅ | `phase-07-strict-csp-hashed.md` |
-| 8 | Audio cache LRU rule | `phase-08-audio-cache-lru.md` |
+| 8 | Audio cache LRU rule ✅ | `phase-08-audio-cache-lru.md` |
 | 9 | PWA install verification checklist | `phase-09-pwa-verify-install.md` |
 
 ## Cuts (YAGNI)

vite.config.js

@@ -61,7 +61,15 @@ export default defineConfig(({ mode }) => {
                 expiration: {
                   // Headroom for future voice growth: 4× current 184 clips.
                   maxEntries: 400,
-                  maxAgeSeconds: 60 * 60 * 24 * 30,
+                  // 7d (was 30d). A clip stays cached as long as it gets
+                  // played at least once a week. Voices that go unused
+                  // for 7d fall out and re-fetch on next play —
+                  // approximates LRU without a custom workbox plugin.
+                  // Each clip <200KB so re-fetch cost is negligible.
+                  maxAgeSeconds: 60 * 60 * 24 * 7,
+                  // Belt-and-suspenders for the rare quota-pressure case
+                  // (lots of voices precached + other origins competing).
+                  purgeOnQuotaError: true,
                 },
                 // Audio is same-origin → no need to allow opaque (0).
                 cacheableResponse: { statuses: [200] },