Add live transcode progress with a dedicated now-processing panel

- ffmpeg progress streamed via -progress pipe:1, persisted on the job as a percent
- retry/retry-all now mark the old row as superseded instead of deleting it,
  preserving the failed/cancelled audit trail
- frontend shows the running job in its own panel above the list with a live
  progress bar instead of burying it as just another row

backend/models.py

@@ -366,6 +366,8 @@ class TranscodeJob(BaseModel):
     status: str = "pending"
     error: str = ""
     triggered_by: str = "manual"
+    progress: float = 0.0
+    superseded_by: Optional[str] = None
     created_at: str = Field(default_factory=_now_iso)
     started_at: Optional[str] = None
     finished_at: Optional[str] = None