fix: add created field to chapters_idx to fix recentlyUpdatedBooks 400

chapters_idx was missing created/updated columns (never defined in the
PocketBase schema), causing PocketBase to return 400 for any query
sorted by -created. recentlyUpdatedBooks() uses this sort.

- Add created date field to chapters_idx schema in pb-init-v3.sh
  (also added via add_field for existing installations)
- Add idx_chapters_idx_created index for sort performance
- Set created timestamp on first insert in upsertChapterIdx so new
  chapters are immediately sortable; existing records retain empty created
  and will sort to the back (acceptable — only affects home page recency)

backend/internal/storage/store.go

@@ -130,7 +130,14 @@ func (s *Store) upsertChapterIdx(ctx context.Context, slug string, ref domain.Ch
 		return err
 	}
 	if len(items) == 0 {
-		postErr := s.pb.post(ctx, "/api/collections/chapters_idx/records", payload, nil)
+		// Set created timestamp on first insert so recentlyUpdatedBooks can sort by it.
+		insertPayload := map[string]any{
+			"slug":    slug,
+			"number":  ref.Number,
+			"title":   ref.Title,
+			"created": time.Now().UTC().Format(time.RFC3339),
+		}
+		postErr := s.pb.post(ctx, "/api/collections/chapters_idx/records", insertPayload, nil)
 		if postErr == nil {
 			return nil
 		}

scripts/pb-init-v3.sh

@@ -149,9 +149,10 @@ create "books" '{
 
 create "chapters_idx" '{
   "name":"chapters_idx","type":"base","fields":[
-    {"name":"slug",  "type":"text",   "required":true},
-    {"name":"number","type":"number", "required":true},
-    {"name":"title", "type":"text"}
+    {"name":"slug",    "type":"text",   "required":true},
+    {"name":"number",  "type":"number", "required":true},
+    {"name":"title",   "type":"text"},
+    {"name":"created", "type":"date"}
   ]}'
 
 create "ranking" '{
@@ -326,9 +327,12 @@ add_field "app_users"      "polar_customer_id"      "text"
 add_field "app_users"      "polar_subscription_id"  "text"
 add_field "user_library"   "shelf"                  "text"
 add_field "user_sessions"  "device_fingerprint"     "text"
+add_field "chapters_idx"   "created"                "date"
 
 # ── 6. Indexes ────────────────────────────────────────────────────────────────
 add_index "chapters_idx" "idx_chapters_idx_slug_number" \
   "CREATE UNIQUE INDEX idx_chapters_idx_slug_number ON chapters_idx (slug, number)"
+add_index "chapters_idx" "idx_chapters_idx_created" \
+  "CREATE INDEX idx_chapters_idx_created ON chapters_idx (created)"
 
 log "done"