DR-LYRIA-001: Waveform performance overhaul

- Convert generate_waveform_svg() from sync to async
  (subprocess.run -> asyncio.create_subprocess_exec)
- Add early downsampling to ~2000 samples before numpy processing
  (~95% memory reduction for 120s tracks)
- Pre-generate waveform on clip completion in _generate_and_save()
- Add Cache-Control headers (public, max-age=86400) to waveform endpoint
- Preserve on-demand generation as lazy fallback

backend/src/main.py

@@ -49,7 +49,11 @@ async def list_models():
     return {"models": LYRIA_MODELS, "default": DEFAULT_MODEL}
 
 async def _generate_and_save(clip_id: str, prompt: str, model: str, duration: int):
-    """Background task: call Lyria API and save the result."""
+    """Background task: call Lyria API and save the result.
+    
+    DR-LYRIA-001: Pre-generates waveform SVG after clip is ready,
+    so users never wait for on-demand generation.
+    """
     try:
         result = await generate_clip(prompt, model, duration)
         
@@ -60,6 +64,16 @@ async def _generate_and_save(clip_id: str, prompt: str, model: str, duration: in
                 status="ready", 
                 file_size=file_info["file_size"]
             )
+            # DR-LYRIA-001: Pre-generate waveform SVG immediately
+            # so it's ready before the user views the clip
+            try:
+                clip = await get_clip(clip_id)
+                if clip and clip.get("filename"):
+                    await generate_waveform_svg(clip_id, clip["filename"])
+            except Exception as wf_err:
+                # Waveform pre-generation failure is non-fatal —
+                # on-demand fallback will catch it on view
+                print(f"Waveform pre-gen warning for {clip_id}: {wf_err}")
         else:
             await update_clip_status(
                 clip_id, 
@@ -228,14 +242,22 @@ async def get_waveform(clip_id: str):
     clip = await get_clip(clip_id)
     if not clip:
         raise HTTPException(status_code=404, detail="Clip not found")
-    # Check if waveform already exists
+    # Check if pre-generated waveform exists on disk
     svg_path = get_waveform_path(clip_id)
     if svg_path and svg_path.exists():
-        return FileResponse(svg_path, media_type="image/svg+xml")
-    # Generate on demand
+        return FileResponse(
+            svg_path, 
+            media_type="image/svg+xml",
+            headers={"Cache-Control": "public, max-age=86400"}
+        )
+    # Lazy fallback: generate on demand (should rarely trigger)
     try:
-        svg_path_str = generate_waveform_svg(clip_id, clip["filename"])
-        return FileResponse(svg_path_str, media_type="image/svg+xml")
+        svg_path_str = await generate_waveform_svg(clip_id, clip["filename"])
+        return FileResponse(
+            svg_path_str, 
+            media_type="image/svg+xml",
+            headers={"Cache-Control": "public, max-age=86400"}
+        )
     except FileNotFoundError:
         raise HTTPException(status_code=404, detail="Audio file not found")
     except Exception as e: