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/pipeline/audio.py

@@ -9,13 +9,13 @@ log = get_logger(__name__)
 
 def extract_audio(video_path: Path, audio_path: Path) -> Path:
     """Extract 16kHz mono wav from any video/audio container."""
-    if not video_path.exists():
-        raise AudioExtractionError(f"Uploaded file not found on disk at {video_path}. It may not have finished uploading, or the upload volume isn't mounted correctly.")
-
     if audio_path.exists():
         log.info("Audio already extracted at %s, skipping", audio_path)
         return audio_path
 
+    if not video_path.exists():
+        raise AudioExtractionError(f"Uploaded file not found on disk at {video_path}. It may not have finished uploading, or the upload volume isn't mounted correctly.")
+
     log.info("Extracting audio: %s -> %s", video_path, audio_path)
     result = subprocess.run(
         [
@@ -33,3 +33,30 @@ def extract_audio(video_path: Path, audio_path: Path) -> Path:
             f"This usually means the file is corrupt or not a supported format. Last output:\n{stderr_tail}"
         )
     return audio_path
+
+
+def generate_playback_audio(video_path: Path, playback_path: Path) -> Path | None:
+    """Generate a compressed Opus file for browser playback (~48 kbps).
+
+    This is done alongside the WAV extraction so the frontend can download a
+    small file for instant-seeking Blob playback.  Existing sessions that lack
+    the Opus file will fall back to the original upload or the extracted WAV.
+    """
+    if playback_path.exists():
+        log.info("Playback audio already exists at %s, skipping", playback_path)
+        return playback_path
+
+    log.info("Generating playback audio: %s -> %s", video_path, playback_path)
+    result = subprocess.run(
+        [
+            "ffmpeg", "-y", "-i", str(video_path),
+            "-vn", "-c:a", "libopus", "-b:a", "48k",
+            str(playback_path),
+        ],
+        capture_output=True, text=True,
+    )
+    if result.returncode != 0:
+        stderr_tail = "\n".join(result.stderr.strip().splitlines()[-5:])
+        log.warning("Failed to generate playback audio (non-fatal): %s", stderr_tail)
+        return None
+    return playback_path