Preload audio as Blob for instant seeking; generate Opus playback file; delete original upload after extraction

- Download full audio as Blob on mount so seeking is instant (no network range-requests)
- Remove readyState guard from playAt() that was silently blocking seek attempts
- Add disabled/ready state to SessionReel for reliable click-to-seek
- Generate 48kbps Opus playback file during extraction (~65MB for 3hrs vs 1.5GB lossless)
- Serve Opus first in audio endpoint, then original upload, then extracted WAV
- Delete original upload after successful transcription to reclaim storage
- Reorder extract_audio checks so retry works without the original file
- Fix Back links on Speakers and Notes pages to go to home

backend/app/routers/sessions.py

@@ -8,7 +8,7 @@ from fastapi import APIRouter, UploadFile, File, Form, HTTPException
 from .. import database as db, config, jobs
 from ..config import merge_campaign_settings_with_env
 from ..errors import PipelineError
-from ..pipeline.audio import extract_audio
+from ..pipeline.audio import extract_audio, generate_playback_audio
 from ..pipeline.transcribe import transcribe_and_diarize
 
 
@@ -96,6 +96,7 @@ def _run_transcription(session_id: str, video_path: Path, audio_path: Path, tran
         progress_cb("Extracting audio...")
         try:
             extract_audio(video_path, audio_path)
+            generate_playback_audio(video_path, audio_path.with_suffix(".opus"))
             transcribe_and_diarize(
                 audio_path,
                 transcript_path,
@@ -114,6 +115,16 @@ def _run_transcription(session_id: str, video_path: Path, audio_path: Path, tran
                 (str(audio_path), str(transcript_path), session_id),
             )
 
+        # Original upload no longer needed — Opus serves playback, WAV serves
+        # re-transcription.  Reclaim storage.
+        if video_path.exists():
+            try:
+                video_path.unlink()
+                with db.tx() as conn:
+                    conn.execute("UPDATE sessions SET video_path = NULL WHERE id = ?", (session_id,))
+            except OSError:
+                pass  # non-fatal
+
     job_id = db.create_job(session_id, "transcribe")
     jobs.submit(job_id, run, requires_gpu=True)
     return job_id