feat(backend): voice extraction endpoint

POST /voice/extract behind JwtAuthGuard + ClinicOrgGuard, plus
GET /voice/availability so the frontend can decide whether to render the
microphone — it cannot learn that from NEXT_PUBLIC_*, which are baked in at
build time.

Audio is held in memory for the request only: never written to disk, never a
Prisma row. The transcript goes back to the client and is not persisted. What
is logged is structured and patient-free — clip length, which fields resolved,
unresolved count, vendor cost, outcome — with log lines as the interim sink
until this repo has metrics infrastructure.

On extraction failure the transcript still travels back in the error details,
so the words the clinician already paid for can be salvaged into a note.

v1 ships ungated beyond a configured locale profile; the Plan.features design
is deferred, not dropped.

From review of this commit, four of which were load-bearing:

- Express's 100 kb default body limit rejected any recording past ~20 seconds,
  making the endpoint unusable at its own 2-minute cap. Body parsers are now
  registered explicitly with a 10 MB limit scoped to the voice route only.
  Verified empirically: 600 KB reaches /api/voice/extract, while /api/auth/login
  still 413s.
- ThrottlerGuard keys on req.ip, so behind nginx the whole deployment would
  share one bucket and an abuser rotating IPs would bypass it. VoiceThrottlerGuard
  keys on the user id instead — with no plan gate, this is the only control on
  metered vendor spend.
- ThrottlerException had no 429 fallback and surfaced as INTERNAL_ERROR; the
  guard now throws VOICE_RATE_LIMITED directly.
- durationMs was optional, so omitting it bypassed VOICE_MAX_RECORDING_MS
  entirely. It is required.
- VOICE_UNSUPPORTED_FORMAT was dead code — the DTO's @IsIn already rejects
  unknown containers — so it is gone rather than left unreachable.

ThrottlerModule is deliberately not bound as a global APP_GUARD: a global
ThrottlerGuard rate-limits every route against every named throttler, which
would have capped the whole API at the voice limit.

All seven remaining VOICE_* codes have errors.* keys in en, fa and nl.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/.env.example

@@ -47,3 +47,28 @@ SMTP_PASSWORD=your_app_password
 # SMS_IR_API_KEY=4QKMiSU4Kh7tWPLCdRMV0QpDh8WgF33YkWRS18BcG3vf4QHi
 SMS_IR_API_KEY=lwbK7hxmjimNjFS4g5DWahh75EKCgJUfcUIinUQzfQXwXkSp
 SMS_IR_TEMPLATE_ID=123456
+
+# ── Voice treatment entry ──────────────────────────────────────────────────────
+# Without OPENROUTER_API_KEY the microphone button does not render at all.
+OPENROUTER_API_KEY=
+# OPENROUTER_BASE_URL=https://openrouter.ai/api/v1
+
+# Locales the microphone is offered in. An unknown locale here fails at boot.
+# VOICE_ENABLED_LOCALES=fa,en,nl
+
+# Models, overridable per locale (VOICE_ASR_MODEL_FA, VOICE_LLM_MODEL_NL, ...).
+# All locales share these today; the per-locale override exists so Persian can be
+# repointed at a specialist ASR vendor without a code change.
+# VOICE_ASR_MODEL=openai/whisper-1
+# VOICE_LLM_MODEL=google/gemini-3.7-flash
+# VOICE_ASR_PROVIDER_FA=openrouter
+# VOICE_LLM_PROVIDER_FA=openrouter
+
+# Recording cap in ms (0 = uncapped). 2 minutes bounds worst-case vendor spend at
+# about 1.3 cents per recording.
+# VOICE_MAX_RECORDING_MS=120000
+
+# Per-user rate limit on the extract endpoint. Unreachable by a human — a recording
+# plus processing takes ten seconds at minimum — so it is purely an abuse guard.
+# VOICE_THROTTLE_TTL=60
+# VOICE_THROTTLE_LIMIT=6