fix(gallery): infinite scroll + timeline + jump use tag_id

All three API calls (/api/gallery/scroll, /api/gallery/timeline,
/api/gallery/jump) were still sending the legacy ?tag=<name> param.
Task 10's backend switch silently ignored it, so subsequent scroll
loads and timeline navigation fetched unfiltered content on a
tag-filtered gallery page.

Read tag_id from window.location.search at init and cache it in
state.activeTagId. Use it as ?tag_id=<int> on every subsequent
fetch. state.activeTag (the display string) stays for human-facing
uses elsewhere.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/static/js/gallery-infinite.js

@@ -20,7 +20,8 @@
     isLoading: false,
     hasMore: true,
     cursor: null,
-    activeTag: null,
+    activeTag: null,             // display string, used for human-facing bits
+    activeTagId: null,           // integer id, authoritative for filter API calls
     loadedSections: new Set(),   // year_month keys of loaded sections
     timelineData: [],
     currentVisibleSection: null,
@@ -53,6 +54,16 @@
       state.activeTag = window.galleryState.activeTag;
     }
 
+    // Read tag_id from the current URL — that's what the backend filtered by
+    // for the initial render, and what the scroll/timeline/jump APIs now
+    // expect after the ?tag=<name> -> ?tag_id=<int> switch.
+    const urlParams = new URLSearchParams(window.location.search);
+    const tagIdRaw = urlParams.get('tag_id');
+    if (tagIdRaw) {
+      const n = parseInt(tagIdRaw, 10);
+      if (!isNaN(n)) state.activeTagId = n;
+    }
+
     // Track initially loaded sections
     parseInitialSections();
 
@@ -93,8 +104,8 @@
       if (state.cursor) {
         params.set('cursor', state.cursor);
       }
-      if (state.activeTag) {
-        params.set('tag', state.activeTag);
+      if (state.activeTagId) {
+        params.set('tag_id', String(state.activeTagId));
       }
 
       const response = await fetch(`/api/gallery/scroll?${params}`);
@@ -289,7 +300,7 @@
   // Timeline functions
   async function loadTimeline() {
     try {
-      const params = state.activeTag ? `?tag=${encodeURIComponent(state.activeTag)}` : '';
+      const params = state.activeTagId ? `?tag_id=${state.activeTagId}` : '';
       const response = await fetch(`/api/gallery/timeline${params}`);
       const data = await response.json();
 
@@ -386,8 +397,8 @@
         limit: CONFIG.BATCH_SIZE
       });
 
-      if (state.activeTag) {
-        params.set('tag', state.activeTag);
+      if (state.activeTagId) {
+        params.set('tag_id', String(state.activeTagId));
       }
 
       const response = await fetch(`/api/gallery/jump?${params}`);