fix(gemini): correct fallback model ID + graceful downloads for failed jobs

- gemini-3.1-flash-preview doesn't exist; replace with gemini-3-flash-preview
- GET /jobs/{id}/downloads: return empty {} instead of 400 when job has no
  outputs (e.g. processing_failed before AI stage completes)

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

backend/app/api/v1/routes_jobs.py

@@ -1370,20 +1370,21 @@ async def get_job_downloads(
         if _ap is not None and not _own and not (_jpid and _jpid in (_ap or [])):
             raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Access denied")
 
-    # Allow downloads for jobs that have outputs available
-    # (PENDING_QC, APPROVED_ENGLISH, TRANSLATING, COMPLETED, etc.)
-    if job_doc["status"] in [JobStatus.CREATED.value, JobStatus.INGESTING.value, JobStatus.AI_PROCESSING.value]:
+    # Block only the statuses where outputs can't possibly exist yet
+    _no_output_statuses = {
+        JobStatus.CREATED.value,
+        JobStatus.INGESTING.value,
+        JobStatus.AI_PROCESSING.value,
+    }
+    if job_doc["status"] in _no_output_statuses:
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
             detail="Job is still being processed"
         )
 
-    # Check if job has outputs
+    # No outputs yet — return empty instead of 400 (e.g. failed jobs with partial state)
     if not job_doc.get("outputs"):
-        raise HTTPException(
-            status_code=status.HTTP_400_BAD_REQUEST,
-            detail="No outputs available for this job"
-        )
+        return JobDownloadsResponse(downloads={})
 
     # Generate signed URLs for all outputs
     downloads = {}

backend/app/services/gemini.py

@@ -45,7 +45,7 @@ async def _record_gemini_usage(
 
 class GeminiService:
     _fallback_models: list[str] = [
-        "gemini-3.1-flash-preview",
+        "gemini-3-flash-preview",
         "gemini-2.5-pro",
     ]