feat(cost): add Whisper transcription cost tracking

Records audio_duration (as chars) + latency_ms to cost tracker after each
successful transcription; wrapped in try/except so it never fails the task.

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

backend/app/tasks/whisper_transcribe.py

@@ -1,6 +1,8 @@
 """Celery task for Whisper transcription with Cloud Run fallback."""
 
+import asyncio
 import os
+import time
 import uuid
 
 import google.auth.transport.requests
@@ -8,9 +10,11 @@ import httpx
 from google.auth import default
 from google.cloud import storage
 from google.oauth2 import id_token
+from motor.motor_asyncio import AsyncIOMotorClient
 
 from ..core.config import settings
 from ..core.logging import get_logger
+from ..services import cost_tracker
 from ..services.whisper_service import whisper_service
 from . import celery_app
 
@@ -180,14 +184,15 @@ def transcribe_video_audio_task(self, job_id: str, audio_path: str) -> dict:
     """
     logger.info(f"Starting Whisper transcription task for job {job_id}")
 
+    t_start = time.monotonic()
     try:
         # Use Cloud Run if configured, otherwise local
         if settings.whisper_service_url:
             logger.info(f"Using Cloud Run Whisper service: {settings.whisper_service_url}")
-            return _transcribe_via_cloud_run(job_id, audio_path)
+            result = _transcribe_via_cloud_run(job_id, audio_path)
         else:
             logger.info("Using local Whisper service")
-            return _transcribe_locally(job_id, audio_path)
+            result = _transcribe_locally(job_id, audio_path)
 
     except httpx.HTTPStatusError as e:
         logger.error(f"Cloud Run transcription failed for job {job_id}: {e.response.status_code} - {e.response.text}")
@@ -195,3 +200,32 @@ def transcribe_video_audio_task(self, job_id: str, audio_path: str) -> dict:
     except Exception as e:
         logger.error(f"Whisper transcription failed for job {job_id}: {e}")
         raise
+
+    latency_ms = int((time.monotonic() - t_start) * 1000)
+    audio_duration = result.get("audio_duration", 0.0)
+    if audio_duration:
+        try:
+            async def _fetch_job():
+                client = AsyncIOMotorClient(settings.mongodb_uri)
+                try:
+                    return await client[settings.mongodb_db].jobs.find_one({"_id": job_id})
+                finally:
+                    client.close()
+
+            job_doc = asyncio.run(_fetch_job())
+            user_id = str(job_doc.get("created_by", "")) if job_doc else ""
+            project_id = str(job_doc.get("cost_tracker_project_id", "")) if job_doc else ""
+            cost_tracker.record(
+                model="whisper-1",
+                provider="openai",
+                user_external_id=user_id,
+                project_id=project_id or None,
+                job_external_id=job_id,
+                chars=int(audio_duration),
+                latency_ms=latency_ms,
+                status="success",
+            )
+        except Exception as e:
+            logger.warning(f"Cost tracking failed for job {job_id} (non-fatal): {e}")
+
+    return result