Live preview in Edit modal — fast low-q clip render

User feedback: 'dejstvo je da trajna ker more najprej zrenderirat? to traja?
za to bi rabili hudo mašino al?'

Solution before GPU upgrade: live preview that renders just the selected
range as low-quality 480p clip. ~2-3s instead of ~70s full reel render.

NEW endpoint: GET /api/preview-clip/{job_id}?start=X&end=Y
- ffmpeg fast extract (no reframe, no subtitles, no face tracking)
- 480p ultrafast x264 preset, CRF 30
- Cached per job+range (re-clicks are instant)
- ~2-3s on CPU

Frontend:
- '▶ Predvajaj odsek' button now triggers preview-clip render
- Shows status: '🎬 Renderiram odsek... (~3s)'
- After render: video element switches to preview src
- User sees EXACTLY what reel will contain (just without face track)
- Subsequent clicks on same range are instant (cached)

Workflow:
- Drag handles → click '▶ Predvajaj odsek' → 3s wait → see + hear it
- Iterate fast: drag → preview → drag → preview
- Final '✅ Shrani in re-render' only when satisfied (~70s full render)

app/main.py

@@ -1249,6 +1249,72 @@ async def source_video(job_id: str, user: str = Depends(check_auth)):
     )
 
 
+@app.get("/api/preview-clip/{job_id}")
+async def preview_clip(
+    job_id: str,
+    start: float,
+    end: float,
+    user: str = Depends(check_auth),
+):
+    """Live preview odseka — low-quality 480p clip za hiter predogled.
+    
+    BREZ reframe (16:9 ostane), BREZ napisov, BREZ face tracking.
+    Cilj: ~2-3s render za takojšen feedback med Edit dragom.
+    
+    Cache po start+end timestampih da ponovljeni request ne renderira ponovno.
+    """
+    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 end <= start:
+        raise HTTPException(400, "end mora biti večji od start")
+    duration = end - start
+    if duration < 1:
+        raise HTTPException(400, "Trajanje vsaj 1s")
+    if duration > 90:
+        raise HTTPException(400, "Trajanje največ 90s")
+    
+    # Cache key — preview se shrani in ne re-renderira če isti range
+    cache_key = f"{job_id}_preview_{start:.1f}_{end:.1f}.mp4"
+    cache_path = OUTPUT_DIR / cache_key
+    
+    if not cache_path.exists():
+        # ffmpeg low-q clip — copy stream če gre, sicer hitro re-encode
+        # -ss pred -i: hiter seek (keyframe), idealno za preview
+        cmd = [
+            "ffmpeg", "-y",
+            "-ss", f"{max(0, start - 0.5):.2f}",  # 0.5s buffer za keyframe
+            "-i", str(src),
+            "-ss", f"{min(0.5, start):.2f}",  # fine seek
+            "-t", f"{duration:.2f}",
+            "-vf", "scale=854:480:force_original_aspect_ratio=decrease",  # 480p
+            "-c:v", "libx264",
+            "-preset", "ultrafast",  # NAJHITREJŠI preset
+            "-crf", "30",  # nižja kvaliteta = hitrejše
+            "-c:a", "aac",
+            "-b:a", "96k",
+            "-movflags", "+faststart",
+            "-loglevel", "error",
+            str(cache_path),
+        ]
+        try:
+            proc = subprocess.run(cmd, capture_output=True, text=True, timeout=20)
+            if proc.returncode != 0 or not cache_path.exists():
+                raise HTTPException(500, f"FFmpeg failed: {proc.stderr[-300:]}")
+        except subprocess.TimeoutExpired:
+            raise HTTPException(500, "Preview render timeout (>20s)")
+    
+    return FileResponse(
+        path=cache_path,
+        media_type="video/mp4",
+        headers={"Accept-Ranges": "bytes", "Cache-Control": "max-age=300"},
+    )
+
+
 @app.get("/api/transcript/{job_id}")
 async def get_transcript(job_id: str, user: str = Depends(check_auth)):
     """Vrne transkript (segmente + words) za editor."""