Add background job queue system for generation

- Implements sequential job queue with background worker thread (_enqueue_job, _queue_worker)
- All generate routes now return job_id instead of prompt_id; frontend polls /api/queue/<id>/status
- Queue management UI in navbar with live badge, job list, pause/resume/remove controls
- Fix: replaced url_for() calls inside finalize callbacks with direct string paths (url_for raises RuntimeError without request context in background threads)
- Batch cover generation now uses two-phase pattern: queue all jobs upfront, then poll concurrently via Promise.all so page navigation doesn't interrupt the process
- Strengths gallery sweep migrated to same two-phase pattern; sgStop() cancels queued jobs server-side
- LoRA weight randomisation via lora_weight_min/lora_weight_max already present in _resolve_lora_weight

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/style.css

@@ -121,6 +121,74 @@ a:hover { color: #9d98ff; }
   opacity: 1;
 }
 
+/* ---- Queue button in navbar ---- */
+.queue-btn {
+  position: relative;
+  background: transparent;
+  border: none;
+  color: var(--text-muted);
+  font-size: 1rem;
+  padding: 0.2rem 0.5rem;
+  border-radius: 6px;
+  transition: color 0.15s, background 0.15s;
+  line-height: 1;
+}
+.queue-btn:hover {
+  color: var(--text);
+  background: rgba(255, 255, 255, 0.07);
+}
+.queue-btn-active {
+  color: var(--accent) !important;
+}
+.queue-badge {
+  position: absolute;
+  top: -2px;
+  right: -4px;
+  background: var(--accent);
+  color: #fff;
+  font-size: 0.6rem;
+  font-weight: 700;
+  min-width: 16px;
+  height: 16px;
+  border-radius: 8px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0 3px;
+  line-height: 1;
+}
+.queue-icon {
+  font-size: 0.95rem;
+}
+
+/* Queue status dots */
+.queue-status-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  flex-shrink: 0;
+  display: inline-block;
+}
+.queue-status-pending    { background: var(--text-muted); }
+.queue-status-processing { background: var(--warning); animation: pulse 1s infinite; }
+.queue-status-paused     { background: var(--text-dim); }
+.queue-status-done       { background: var(--success); }
+.queue-status-failed     { background: var(--danger); }
+.queue-status-removed    { background: var(--border); }
+
+@keyframes pulse {
+  0%, 100% { opacity: 1; }
+  50% { opacity: 0.4; }
+}
+
+/* Small button variant for queue actions */
+.btn-xs {
+  padding: 0.1rem 0.35rem;
+  font-size: 0.7rem;
+  border-radius: 4px;
+  line-height: 1.4;
+}
+
 /* ============================================================
    Cards
    ============================================================ */