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/models.py

@@ -4,6 +4,26 @@ from django.db import models
 import uuid
 
 
+TAG_CLOUD_CACHE_KEYS = [
+    "j621.library.tag_cloud.auth",
+    "j621.library.tag_cloud.guest",
+]
+
+
+def build_search_tags(tags, e621_data):
+    """Lowercase tag text (custom + e621) used for library tag search."""
+    names = [str(tag) for tag in (tags or [])]
+    data = e621_data if isinstance(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)
+    return " ".join(
+        sorted({str(name).strip().lower() for name in names if str(name).strip()})
+    )
+
+
 class MediaItem(models.Model):
     """A logical media file, identified by its MD5 fingerprint."""
 
@@ -29,6 +49,9 @@ class MediaItem(models.Model):
     dhash = models.CharField(max_length=32, blank=True, default="", db_index=True)
     phash = models.CharField(max_length=32, blank=True, default="", db_index=True)
     whash = models.CharField(max_length=32, blank=True, default="", db_index=True)
+    # Derived search/filter helpers: flattened tag text and custom-data flag.
+    search_tags = models.TextField(blank=True, default="")
+    has_custom_data = models.BooleanField(default=False, db_index=True)
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)
 
@@ -39,13 +62,22 @@ class MediaItem(models.Model):
         return f"J-{self.pk} ({self.md5})"
 
     def save(self, *args, **kwargs):
+        from django.core.cache import cache
+
         from .guest_filter import item_is_hidden_for_guests
 
         self.hidden_from_guests = item_is_hidden_for_guests(self)
+        self.has_custom_data = bool(self.tags or self.notes)
+        self.search_tags = build_search_tags(self.tags, self.e621_data)
         update_fields = kwargs.get("update_fields")
         if update_fields is not None:
-            kwargs["update_fields"] = set(update_fields) | {"hidden_from_guests"}
+            kwargs["update_fields"] = set(update_fields) | {
+                "hidden_from_guests",
+                "has_custom_data",
+                "search_tags",
+            }
         super().save(*args, **kwargs)
+        cache.delete_many(TAG_CLOUD_CACHE_KEYS)
 
 
 class MediaLocation(models.Model):