feat: Celery setup and transcription task — Whisper + FFmpeg pipeline

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

backend/app/schemas/transcription.py

@@ -0,0 +1,23 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel
+
+
+class TranscriptionSegment(BaseModel):
+    start: float
+    end: float
+    text: str
+
+
+class TranscriptionResponse(BaseModel):
+    id: uuid.UUID
+    recording_id: uuid.UUID
+    text: str
+    segments: list[TranscriptionSegment]
+    language: str | None
+    whisper_model: str
+    processing_time: float | None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}