Decouple PageConfig cross-domain queries + merge cart into db_market

PageConfig (db_blog) decoupling:
- Blog: add page-config, page-config-by-id, page-configs-batch data endpoints
- Blog: add update-page-config action endpoint for events payment admin
- Cart: hydrate_page, resolve_page_config, get_cart_grouped_by_page all
  fetch PageConfig from blog via HTTP instead of direct DB query
- Cart: check_sumup_status auto-fetches page_config from blog when needed
- Events: payment routes read/write PageConfig via blog HTTP endpoints
- Order model: remove cross-domain page_config ORM relationship (keep column)

Cart + Market DB merge:
- Cart tables (cart_items, orders, order_items) moved into db_market
- Cart app DATABASE_URL now points to db_market (same bounded context)
- CartItem.product / CartItem.market_place relationships work again
  (same database, no cross-domain join issues)
- Updated split-databases.sh, init-databases.sql, docker-compose.yml

Ghost sync fix:
- Wrap PostAuthor/PostTag delete+re-add in no_autoflush block
- Use synchronize_session="fetch" to keep identity map consistent
- Prevents query-invoked autoflush IntegrityError on composite PK

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_config/init-databases.sql

@@ -6,7 +6,6 @@
 
 CREATE DATABASE db_account;
 CREATE DATABASE db_blog;
-CREATE DATABASE db_market;
-CREATE DATABASE db_cart;
+CREATE DATABASE db_market;   -- also houses cart tables (commerce bounded context)
 CREATE DATABASE db_events;
 CREATE DATABASE db_federation;

_config/split-databases.sh

@@ -64,14 +64,14 @@ DB_TABLES[db_market]="
   link_errors
   link_externals
   subcategory_redirects
-"
-
-DB_TABLES[db_cart]="
   cart_items
   orders
   order_items
 "
 
+# db_cart merged into db_market — cart and market share the same bounded context
+# (commerce). Cart needs direct read access to products/market_places.
+
 DB_TABLES[db_events]="
   calendars
   calendar_slots
@@ -99,7 +99,7 @@ DB_TABLES[db_federation]="
 
 # ── Migrate each domain ────────────────────────────────────────────────────
 
-for target_db in db_account db_blog db_market db_cart db_events db_federation; do
+for target_db in db_account db_blog db_market db_events db_federation; do
   tables="${DB_TABLES[$target_db]}"
   table_list=""
   for t in $tables; do
@@ -120,7 +120,7 @@ done
 
 echo ""
 echo "=== Stamping Alembic head in each DB ==="
-for target_db in db_account db_blog db_market db_cart db_events db_federation; do
+for target_db in db_account db_blog db_market db_events db_federation; do
   # Create alembic_version table and stamp current head
   psql -q "$target_db" <<'SQL'
     CREATE TABLE IF NOT EXISTS alembic_version (
@@ -144,7 +144,7 @@ echo ""
 echo "Per-app DATABASE_URL values:"
 echo "  blog:       postgresql+asyncpg://postgres:change-me@db:5432/db_blog"
 echo "  market:     postgresql+asyncpg://postgres:change-me@db:5432/db_market"
-echo "  cart:       postgresql+asyncpg://postgres:change-me@db:5432/db_cart"
+echo "  cart:       postgresql+asyncpg://postgres:change-me@db:5432/db_market  (shared with market)"
 echo "  events:     postgresql+asyncpg://postgres:change-me@db:5432/db_events"
 echo "  federation: postgresql+asyncpg://postgres:change-me@db:5432/db_federation"
 echo "  account:    postgresql+asyncpg://postgres:change-me@db:5432/db_account"