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/main.py

@@ -84,13 +84,21 @@ def get_audio(session_id: str):
     if not row:
         raise HTTPException(404, "Session not found")
 
-    # Prefer the original uploaded file — it's a compressed format the browser
-    # can seek in efficiently.  Fall back to the extracted WAV.
+    # Priority: Opus playback > original upload > extracted WAV.
+    # The Opus file is tiny (~65 MB for 3 hours) so the frontend's Blob
+    # download completes in seconds on any modern connection.
     candidates = []
+
+    if row["audio_path"]:
+        opus = Path(row["audio_path"]).with_suffix(".opus")
+        if opus.exists():
+            candidates.append(opus)
+
     if row["video_path"]:
         p = Path(row["video_path"])
         if p.exists():
             candidates.append(p)
+
     if row["audio_path"]:
         p = Path(row["audio_path"])
         if p.exists():