Library search upgrades: tag search, tag cloud, status filter

Backend:
- MediaItem gains search_tags (custom + e621 tags, lowercase) and
  has_custom_data, maintained on save with a data migration backfill
- File list search accepts search_type=filename|tags|both (tag search is
  word-AND across the flattened tag text) and status=matched|custom|
  unknown filters
- New /api/tags/cloud/ endpoint (cached 2 min per guest/auth, invalidated
  on item changes and deletions) returning the most-used tags, honouring
  guest visibility

Frontend:
- Library sidebar: Filename/Tags/Both selector, status pill toggles
  (persisted), and a clickable tag cloud that runs a tag search
- Roadmap updated

backend/apps/library/migrations/0008_mediaitem_has_custom_data_mediaitem_search_tags.py

@@ -0,0 +1,43 @@
+# Generated by Django 6.1.1 on 2026-09-17 18:04
+
+from django.db import migrations, models
+
+
+def backfill_derived_fields(apps, schema_editor):
+    MediaItem = apps.get_model("library", "MediaItem")
+    for item in MediaItem.objects.all().iterator(chunk_size=200):
+        names = [str(tag) for tag in (item.tags or [])]
+        data = item.e621_data if isinstance(item.e621_data, dict) else {}
+        categories = data.get("tags")
+        if isinstance(categories, dict):
+            for values in categories.values():
+                if isinstance(values, list):
+                    names.extend(str(tag) for tag in values)
+        search_tags = " ".join(
+            sorted({str(name).strip().lower() for name in names if str(name).strip()})
+        )
+        MediaItem.objects.filter(pk=item.pk).update(
+            search_tags=search_tags,
+            has_custom_data=bool(item.tags or item.notes),
+        )
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('library', '0007_tempupload_visual_matches'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='mediaitem',
+            name='has_custom_data',
+            field=models.BooleanField(db_index=True, default=False),
+        ),
+        migrations.AddField(
+            model_name='mediaitem',
+            name='search_tags',
+            field=models.TextField(blank=True, default=''),
+        ),
+        migrations.RunPython(backfill_derived_fields, migrations.RunPython.noop),
+    ]