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

templates/index.html

@@ -1027,7 +1027,8 @@
           <button class="modal-close" title="Zapri (ESC)">×</button>
           <div class="modal-title" style="margin-bottom:12px;">✏️ Edit: ${escapeHtml(title)}</div>
           
-          <video id="edit-video" src="/api/source-video/${jobId}" controls preload="metadata" style="width:100%; max-height:50vh; background:#000; border-radius:6px;"></video>
+          <video id="edit-video" src="/api/source-video/${jobId}?quality=low" controls preload="auto" style="width:100%; max-height:50vh; background:#000; border-radius:6px;"></video>
+          <div id="source-status" style="font-size:11px; color:var(--muted); margin-top:4px; text-align:center;">⏳ Pripravljam predogled (~5s prvič, potem instant)…</div>
           
           <!-- iPhone-style trim bar -->
           <div id="trim-bar" style="position:relative; height:72px; width:100%; flex-shrink:0; box-sizing:border-box; margin-top:18px; background:#1a1a1a; border:2px solid #444; border-radius:8px; overflow:hidden; user-select:none; touch-action:none;">
@@ -1222,72 +1223,36 @@
         }
       };
       
-      // ─── LIVE PREVIEW: naloži low-q clip označenega dela ───
-      window.previewSelection = async function() {
-        const status = document.getElementById("preview-status");
-        const playBtn = document.getElementById("preview-btn");
-        const start = trimStart;
-        const end = trimEnd;
-        
-        if (end - start < 1) {
-          alert("Trajanje vsaj 1s");
-          return;
-        }
-        
-        playBtn.disabled = true;
-        playBtn.textContent = "⏳ Renderiram preview...";
-        status.textContent = `🎬 Renderiram odsek ${formatTime(start)} - ${formatTime(end)} (~3s)...`;
-        
-        const url = `/api/preview-clip/${jobId}?start=${start.toFixed(1)}&end=${end.toFixed(1)}`;
-        const t0 = Date.now();
-        
-        try {
-          // Preverim ali je preview že cached (HEAD request)
-          const headRes = await fetch(url, { method: "HEAD" });
-          if (!headRes.ok) {
-            // Cache miss → trigger render z GET
-            const getRes = await fetch(url);
-            if (!getRes.ok) {
-              const err = await getRes.json().catch(() => ({}));
-              status.textContent = "❌ " + (err.detail || "Render failed");
-              playBtn.disabled = false;
-              playBtn.textContent = "▶ Predvajaj odsek";
-              return;
-            }
-          }
-          
-          const elapsed = ((Date.now() - t0) / 1000).toFixed(1);
-          status.textContent = `✅ Pripravljen v ${elapsed}s. Predvajam...`;
-          
-          // Set video src to preview clip
-          video.src = url;
-          video.load();
-          video.addEventListener("loadeddata", function loadedOnce() {
-            video.removeEventListener("loadeddata", loadedOnce);
-            video.currentTime = 0;
-            video.play();
-          }, { once: true });
-          
-          // Auto-pojavi success
-          setTimeout(() => { status.textContent = ""; }, 3000);
-        } catch (e) {
-          status.textContent = "❌ Napaka: " + e.message;
-        } finally {
-          playBtn.disabled = false;
-          playBtn.textContent = "▶ Predvajaj odsek";
-        }
+      // ─── INSTANT PREVIEW: predvajaj označen del (od trimStart, auto-stop pri trimEnd) ───
+      window.previewSelection = function() {
+        if (!video) return;
+        // Skoči na trim start in predvajaj
+        video.currentTime = trimStart;
+        video.play().catch(err => {
+          console.warn("Play failed:", err);
+        });
       };
       
-      // Auto-stop predvajanje ko doseže trimEnd (samo če gledamo full video)
+      // Auto-stop ko doseže trimEnd (brez render-a, brez preview clip URL)
+      // Samo če NI v aktivnem dragu (ker drag = naročiteljsko seekanje)
       if (video) {
         video.addEventListener("timeupdate", () => {
-          // Če gledamo preview clip (src vsebuje '/api/preview-clip/'), ne ustavi
-          if (video.src && video.src.indexOf("/api/preview-clip/") !== -1) return;
+          if (dragging) return;  // Ne ustavi med dragom
           if (!video.paused && video.currentTime >= trimEnd) {
             video.pause();
             video.currentTime = trimStart;
           }
         });
+        
+        // Source-status: skrij ko se naloži
+        const sourceStatus = document.getElementById("source-status");
+        video.addEventListener("loadeddata", () => {
+          if (sourceStatus) sourceStatus.textContent = "✅ Predogled pripravljen — drag ročajev za fine-tune";
+          setTimeout(() => { if (sourceStatus) sourceStatus.style.display = "none"; }, 2000);
+        });
+        video.addEventListener("error", () => {
+          if (sourceStatus) sourceStatus.textContent = "❌ Napaka pri nalaganju predogleda";
+        });
       }
       
       // Initial render — počakaj da DOM ima dimenzije (modal je bil pravkar dodan)