Add sync clock entity for synchronized animation timing

Introduces Synchronization Clocks — shared, controllable time bases
that CSS sources can optionally reference for synchronized animation.

Backend:
- New SyncClock dataclass, JSON store, Pydantic schemas, REST API
- Runtime clock with thread-safe pause/resume/reset and speed control
- Ref-counted runtime pool with eager creation for API control
- clock_id field on all ColorStripSource types
- Stream integration: clock time/speed replaces source-local values
- Paused clock skips rendering (saves CPU + stops frame pushes)
- Included in backup/restore via STORE_MAP

Frontend:
- Sync Clocks tab in Streams section with cards and controls
- Clock dropdown in CSS editor (hidden speed slider when clock set)
- Clock crosslink badge on CSS source cards (replaces speed badge)
- Targets tab uses DataCache for picture/audio sources and sync clocks

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

server/src/wled_controller/static/js/features/targets.js

@@ -9,6 +9,7 @@ import {
     kcWebSockets,
     ledPreviewWebSockets,
     _cachedValueSources, valueSourcesCache,
+    streamsCache, audioSourcesCache, syncClocksCache,
 } from '../core/state.js';
 import { API_BASE, getHeaders, fetchWithAuth, escapeHtml, isOpenrgbDevice } from '../core/api.js';
 import { t } from '../core/i18n.js';
@@ -475,15 +476,17 @@ export async function loadTargetsTab() {
     if (!csDevices.isMounted()) setTabRefreshing('targets-panel-content', true);
 
     try {
-        // Fetch devices, targets, CSS sources, picture sources, pattern templates, and value sources in parallel
-        const [devicesResp, targetsResp, cssResp, psResp, patResp, valueSrcArr, asResp] = await Promise.all([
+        // Fetch devices, targets, CSS sources, pattern templates in parallel;
+        // use DataCache for picture sources, audio sources, value sources, sync clocks
+        const [devicesResp, targetsResp, cssResp, patResp, psArr, valueSrcArr, asSrcArr] = await Promise.all([
             fetchWithAuth('/devices'),
             fetchWithAuth('/picture-targets'),
             fetchWithAuth('/color-strip-sources').catch(() => null),
-            fetchWithAuth('/picture-sources').catch(() => null),
             fetchWithAuth('/pattern-templates').catch(() => null),
+            streamsCache.fetch().catch(() => []),
             valueSourcesCache.fetch().catch(() => []),
-            fetchWithAuth('/audio-sources').catch(() => null),
+            audioSourcesCache.fetch().catch(() => []),
+            syncClocksCache.fetch().catch(() => []),
         ]);
 
         const devicesData = await devicesResp.json();
@@ -499,10 +502,7 @@ export async function loadTargetsTab() {
         }
 
         let pictureSourceMap = {};
-        if (psResp && psResp.ok) {
-            const psData = await psResp.json();
-            (psData.streams || []).forEach(s => { pictureSourceMap[s.id] = s; });
-        }
+        psArr.forEach(s => { pictureSourceMap[s.id] = s; });
 
         let patternTemplates = [];
         let patternTemplateMap = {};
@@ -516,10 +516,7 @@ export async function loadTargetsTab() {
         valueSrcArr.forEach(s => { valueSourceMap[s.id] = s; });
 
         let audioSourceMap = {};
-        if (asResp && asResp.ok) {
-            const asData = await asResp.json();
-            (asData.sources || []).forEach(s => { audioSourceMap[s.id] = s; });
-        }
+        asSrcArr.forEach(s => { audioSourceMap[s.id] = s; });
 
         // Fetch all device states, target states, and target metrics in batch
         const [batchDevStatesResp, batchTgtStatesResp, batchTgtMetricsResp] = await Promise.all([