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>

frontend/messages/en.json

@@ -1235,6 +1235,13 @@
     "LAB_CASE_START_INCOMPLETE": "Complete teeth and prosthesis types before starting this case.",
     "LAB_CASE_CLIENT_REQUIRED": "Enter a clinic name or a patient name for this case.",
     "BAD_REQUEST": "The request could not be processed.",
-    "INTERNAL_ERROR": "Something went wrong on our end. Please try again later."
+    "INTERNAL_ERROR": "Something went wrong on our end. Please try again later.",
+    "VOICE_MIC_DENIED": "Microphone access was blocked. Allow it in your browser settings and try again.",
+    "VOICE_NOT_AVAILABLE": "Voice entry is not available for this language yet.",
+    "VOICE_CLIP_TOO_LONG": "That recording is too long. Please keep it under two minutes.",
+    "VOICE_ASR_FAILED": "Could not turn the recording into text. Please try again.",
+    "VOICE_EXTRACT_FAILED": "Could not read the treatment details from the recording.",
+    "VOICE_NOTHING_RECOGNIZED": "No speech was recognised. Check the microphone and try again.",
+    "VOICE_RATE_LIMITED": "Too many recordings in a short time. Please wait a moment."
   }
 }