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)

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"