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/common/errors/error-codes.ts

@@ -187,6 +187,7 @@ export const ErrorCode = {
   // Voice treatment entry
   VOICE_NOT_AVAILABLE: 'VOICE_NOT_AVAILABLE',
   VOICE_CLIP_TOO_LONG: 'VOICE_CLIP_TOO_LONG',
+  VOICE_UNSUPPORTED_FORMAT: 'VOICE_UNSUPPORTED_FORMAT',
   VOICE_ASR_FAILED: 'VOICE_ASR_FAILED',
   VOICE_EXTRACT_FAILED: 'VOICE_EXTRACT_FAILED',
   VOICE_NOTHING_RECOGNIZED: 'VOICE_NOTHING_RECOGNIZED',