feat(frontend): filter recordings per-speaker + export WAV for Audacity

- recordings page: speaker filter dropdown (server-side userId filter via
  existing recordings.list proc) + clear-filter + pagination reset on change
- WAV export per card: decode download_url via Web Audio, re-encode 16-bit PCM
  WAV (Audacity-ready) client-side, no server/ffmpeg needed
- EXPORT WAV (N) header button: concatenate visible (filtered) recordings
  into a single mono WAV for mixdown/analysis
- hooks: per-filter SWR cache key (recordings/<userId|all>); delete & live
  WS-sync invalidate every filter cache via key matcher
- new lib/audio/wav.ts: decodeAudio / audioBufferToWav / concatAudioBuffers /
  downloadBlob

.hermes/plans/2026-08-30_recordings-filter-export-wav-spec.md

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+# Spec: Recordings — filter per user + export WAV (Audacity)
+
+## Konteks / Gejala
+Halaman `services/frontend/src/app/(dashboard)/recordings` menampilkan semua
+rekaman voice (deck). User ingin:
+1. **Filter per orang** (tampil rekaman satu user saja).
+2. **Export ke format untuk Audacity** (buka & edit rekaman di Audacity).
+
+## Fakta saat ini (verified)
+- Backend `recordings.list` (services/backend/src/orpc/router.ts:291) SUDAH
+  menerima `userId`/`channelId` filter → `RecordingsService.getRecent`.
+- Frontend `recordingsApi.list(limit, channelId, userId, cursor)` (lib/api/recordings.ts)
+  sudah meneruskan `userId`. `useLoadMoreRecordings` juga sudah bawa userId.
+- Tapi UI `RecordingsView` (app/(dashboard)/recordings/view.tsx) TIDAK punya
+  filter UI, dan `useRecordingsPage` dipanggil tanpa userId → semua tampil.
+- Setiap rekaman punya `download_url` (MP3 di TeleUploader), `user_id`, `username`.
+- Audacity membuka MP3/OGG tapi editing paling bersih dari WAV (uncompressed)
+  / FLAC (lossless). Backend TIDAK punya ffmpeg & Nix flake backend tak include
+  ffmpeg → transcode server-side bukan pilihan. Browser punya codec MP3 → export
+  WAV via Web Audio API (client-side) adalah solusi self-contained terbaik.
+
+## Keputusan desain
+1. **Filter per user**: UI dropdown (Semua User + per user) di header halaman.
+   Memilih user → re-fetch `recordingsApi.list(50, undefined, userId)` (server
+   filter, benar untuk dataset besar + pagination). Dropdown dibangun dari
+   distinct `user_id`/`username` pada items yang sedang tampil.
+2. **Export WAV (Audacity)**: client-side via Web Audio API.
+   - Per kartu: tombol "WAV" → decode `download_url` → WAV 16-bit PCM → download.
+   - Header: tombol "EXPORT WAV (N)" → gabung (concat) semua rekaman yang
+     sedang tampil (ter-filter) jadi 1 file WAV → download. Ideal untuk analisis
+     / mixdown per orang.
+   - Implementasi di `lib/audio/wav.ts` (decode + encode + concat), tanpa dep baru.
+
+## Perubahan
+
+### Frontend
+- **`src/lib/audio/wav.ts`** (baru):
+  - `decodeAudio(url: string): Promise<AudioBuffer>` — fetch arrayBuffer →
+    `new AudioContext().decodeAudioData`.
+  - `audioBufferToWav(buf: AudioBuffer, sampleRate=48000): Blob` — PCM 16-bit
+    interleaved, mono→stereo handling, RIFF/WAVE writer. Audacity-importable.
+  - `concatBuffers(buffers: AudioBuffer[]): AudioBuffer` — gabung di channel 0
+    (mono) dengan sample-rate max; untuk export gabungan.
+  - `downloadWav(blob: Blob, filename: string): void` — obj URL + <a download>.
+- **`src/app/(dashboard)/recordings/view.tsx`**:
+  - Toolbar filter: dropdown user (built from distinct items) + tombol reset.
+  - State `filterUserId`; saat berubah → `recordingsApi.list(50, undefined, id)`
+    → set ke SWR (key includes filter), reset pagination.
+  - Tombol "WAV" per kartu (disabled jika `!r.download_url`).
+  - Tombol "EXPORT WAV (N)" di header (disabled jika 0 item punya download_url);
+    concat semua items ter-filter yang punya download_url.
+  - Status loading saat export (spinner/disable).
+- **`src/hooks/use-recordings.ts`**: `useRecordingsPage` menerima `userId?` dan
+  memasukkan ke key + call, supaya filter re-fetch bersih (per-user cache key).
+  `useRecordings`/`useLoadMoreRecordings` propagate `userId`.
+- **`src/lib/types/recording.ts`**: tidak berubah (userId dari items).
+
+### Backend / gateway
+- Tidak ada perubahan. Filter & export sepenuhnya frontend.
+
+## File yang disentuh (frontend only)
+- `src/lib/audio/wav.ts` (baru)
+- `src/app/(dashboard)/recordings/view.tsx`
+- `src/hooks/use-recordings.ts`
+
+## Verification
+1. `cd services/frontend && pnpm typecheck` (tsc --noEmit) — 0 error.
+2. `pnpm lint` (biome check src/) — exit 0.
+3. `pnpm build` (next build) — hijau.
+4. Manual (user): buka /recordings; pilih user di dropdown → hanya rekaman user
+   itu; klik WAV di kartu → file .wav ter-download & terbuka di Audacity; klik
+   EXPORT WAV (filtered) → satu .wav gabungan.
+5. Push → CI `Build & Deploy (Nix)` (frontend job) hijau → deploy landing.
+
+## Risiko / Trade-off
+- Web Audio decode MP3 di client: butuh CORS pada download_url (TeleUploader
+  asepharyana.my.id — sudah same-serve/proxied, CORS ikut origin). Jika 403/CORS
+  gagal, error toaster + fallback manual (RAW MP3 tetap ada).
+- concat gabungan = mono 48k; Audacity bisa edit per-channel nanti. Acceptable.
+- Filter client (dropdown dari items yang dimuat) hanya menawarkan user yang
+  sudah tampil; dataset besar bisa pakai search nanti. Server filter benar untuk
+  yang dipilih.