Serve original uploaded file instead of WAV for audio playback

The audio endpoint was serving the extracted WAV (16kHz mono PCM)
which balloons to ~1GB for a 3-hour session. Browsers cannot seek
efficiently in such a large uncompressed file.

Now the endpoint serves the original uploaded file (m4a, mp3, opus, etc.)
— a compressed format the browser can seek via byte-range requests.
Falls back to the WAV if the original is missing.

backend/app/main.py

@@ -60,15 +60,49 @@ app.include_router(files_router.router)
 app.include_router(campaigns_router.router)
 
 
+MEDIA_TYPES = {
+    ".mp3": "audio/mpeg",
+    ".mp4": "audio/mp4",
+    ".m4a": "audio/mp4",
+    ".m4b": "audio/mp4",
+    ".ogg": "audio/ogg",
+    ".opus": "audio/ogg",
+    ".wav": "audio/wav",
+    ".flac": "audio/flac",
+    ".aac": "audio/aac",
+    ".webm": "audio/webm",
+}
+
+AUDIO_EXTS = frozenset(MEDIA_TYPES.keys())
+
+
 @app.get("/api/sessions/{session_id}/audio")
 def get_audio(session_id: str):
-    row = db.get_conn().execute("SELECT audio_path FROM sessions WHERE id = ?", (session_id,)).fetchone()
-    if not row or not row["audio_path"]:
-        raise HTTPException(404, "Session not found or audio not available")
-    audio_path = Path(row["audio_path"])
-    if not audio_path.exists():
-        raise HTTPException(404, "Audio not available for this session")
-    return FileResponse(audio_path, media_type="audio/wav")
+    row = db.get_conn().execute(
+        "SELECT video_path, audio_path FROM sessions WHERE id = ?", (session_id,)
+    ).fetchone()
+    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.
+    candidates = []
+    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():
+            candidates.append(p)
+
+    if not candidates:
+        raise HTTPException(404, "No audio file available for this session")
+
+    chosen = candidates[0]
+    ext = chosen.suffix.lower()
+    media_type = MEDIA_TYPES.get(ext, "application/octet-stream")
+    return FileResponse(chosen, media_type=media_type)
 
 
 @app.get("/api/health")