fix(backend): stop showing the clinician null, NaN and the wrong failure

Two ways a voice failure described itself wrongly.

describe() built the quoted-back text from fields that are all nullable on
the wire, and toVoiceIntent casts rather than checks — so a half-classified
deadline rendered as “null null” — not a usable date, and an offset with no
amount as “+NaN day”. Blank is already handled by the sheet; it now falls
back to that.

The DTO's constraints resolved to unrelated codes: maxLength fell through
to VALIDATION_FIELD_REQUIRED, so an oversized recording said a field was
missing, and isIn maps to VALIDATION_LANGUAGE_INVALID, so an unsupported
container said the language was invalid. Both now name their own code —
the validation factory already returns a message verbatim when it is itself
a known ErrorCode, so this needs no change to the shared mapping.

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

backend/src/modules/voice/dto/voice.dto.ts

@@ -6,6 +6,7 @@ import {
   MaxLength,
   Min,
 } from 'class-validator';
+import { ErrorCode } from '../../../common/errors/error-codes';
 
 /** Containers OpenRouter's transcription endpoint accepts, and MediaRecorder can produce. */
 export const VOICE_AUDIO_FORMATS = [
@@ -32,10 +33,14 @@ export class ExtractVoiceDto {
    */
   @IsString()
   @IsBase64()
-  @MaxLength(8_000_000)
+  // Both constraints name their own code. Left to the default mapping, `maxLength` falls
+  // through to VALIDATION_FIELD_REQUIRED and `isIn` resolves to
+  // VALIDATION_LANGUAGE_INVALID — so an oversized recording told the clinician a field
+  // was missing, and an unsupported container told them their language was invalid.
+  @MaxLength(8_000_000, { message: ErrorCode.VOICE_CLIP_TOO_LONG })
   audio: string;
 
-  @IsIn(VOICE_AUDIO_FORMATS)
+  @IsIn(VOICE_AUDIO_FORMATS, { message: ErrorCode.VOICE_UNSUPPORTED_FORMAT })
   format: VoiceAudioFormat;
 
   /**