Edit: instant client-side preview with low-q source

User feedback: 'predvaja odsek in začne iz nule kar ni ok, ne moremo
premikati levo dolj levo... za to bi rabili low-q?'

REPLACED render-on-demand approach with low-q source download:

1. Backend: GET /api/source-video/{id}?quality=low
   - 480p re-encode of full source (cached after first request)
   - veryfast preset, CRF 28
   - First request: ~5-10s (depends on song length)
   - Subsequent: instant (cached)

2. Frontend: Edit modal loads ?quality=low
   - 'Pripravljam predogled (~5s prvič, potem instant)' status
   - Once loaded: ALL preview is client-side instant
   - 'Predvajaj odsek' jumps to trimStart and plays
   - Auto-stop at trimEnd (loops back)
   - Drag handles DURING playback = instant seek (browser scrubs in 5MB)
   - Drag NOT blocked during play (you can fine-tune in/out live)

3. Removed old /api/preview-clip endpoint logic (no longer needed)
   Note: kept the route as cache cleanup for old jobs

Workflow now:
- Open Edit → 5s wait first time
- Drag handles freely (instant scrubbing)
- Click Predvajaj → starts at trimStart immediately
- Drag handles WHILE playing → live preview
- Save when satisfied → 70s full render

app/main.py

@@ -1233,14 +1233,58 @@ async def delete_job(job_id: str, user: str = Depends(check_auth)):
 
 # ─── EDIT FEATURE ────────────────────────────────────────────────
 @app.get("/api/source-video/{job_id}")
-async def source_video(job_id: str, user: str = Depends(check_auth)):
-    """Vrne 16:9 original video za predogled v editorju."""
+async def source_video(job_id: str, quality: str = "high", user: str = Depends(check_auth)):
+    """Vrne 16:9 original video za predogled v editorju.
+    
+    quality='high' (default): originalni file
+    quality='low': 480p re-encoded version (cached) — za hitro scrubbanje v editorju
+    """
     job = load_job(job_id)
     if not job:
         raise HTTPException(404, "Ne obstaja")
     src = job.get("input_path")
     if not src or not Path(src).exists():
         raise HTTPException(404, "Original video ne obstaja")
+    
+    if quality == "low":
+        # 480p cached za hitro scrubbanje
+        cache_path = OUTPUT_DIR / f"{job_id}_source_low.mp4"
+        cache_valid = cache_path.exists() and cache_path.stat().st_size > 1024
+        
+        if not cache_valid:
+            if cache_path.exists():
+                cache_path.unlink()
+            cmd = [
+                "ffmpeg", "-y",
+                "-i", str(src),
+                "-vf", "scale=854:480:force_original_aspect_ratio=decrease,scale=trunc(iw/2)*2:trunc(ih/2)*2",
+                "-c:v", "libx264",
+                "-preset", "veryfast",  # malo boljša kvaliteta od ultrafast
+                "-crf", "28",
+                "-c:a", "aac",
+                "-b:a", "96k",
+                "-movflags", "+faststart",
+                "-loglevel", "error",
+                str(cache_path),
+            ]
+            try:
+                proc = subprocess.run(cmd, capture_output=True, text=True, timeout=120)
+                if proc.returncode != 0 or not cache_path.exists() or cache_path.stat().st_size < 1024:
+                    if cache_path.exists():
+                        cache_path.unlink()
+                    raise HTTPException(500, f"FFmpeg failed: {(proc.stderr or 'unknown')[-300:]}")
+            except subprocess.TimeoutExpired:
+                if cache_path.exists():
+                    cache_path.unlink()
+                raise HTTPException(500, "Low-q source render timeout (>120s)")
+        
+        return FileResponse(
+            path=cache_path,
+            media_type="video/mp4",
+            filename=f"source_low_{job_id}.mp4",
+            headers={"Accept-Ranges": "bytes", "Cache-Control": "max-age=3600"},
+        )
+    
     return FileResponse(
         path=src,
         media_type="video/mp4",