fix(schema): empty string → None for captions/AD VTT fields (Bug 2B)

Frontend sends audio_description_vtt: "" for CC-only jobs.
Pydantic validator converts "" to None before validation,
so the backend skips VTT format validation and returns 200
instead of 400.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/job.py

@@ -1,6 +1,6 @@
 from typing import Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, field_validator
 
 from ..models.job import (
     AccessibleVideoProgressItem,
@@ -78,6 +78,11 @@ class VttUpdateRequest(BaseModel):
     if_match: str | None = None  # Optimistic locking — SHA1 of expected current content
     retranslate_languages: bool = False  # Re-translate all target languages from updated source VTT
 
+    @field_validator('captions_vtt', 'audio_description_vtt', mode='before')
+    @classmethod
+    def empty_str_to_none(cls, v: Any) -> str | None:
+        return None if v == '' else v
+
 
 class VttTimingAdjustRequest(BaseModel):
     offset_seconds: float