cms: depend on openbureau-core (vendored via git subtree at cms/core)

The CMS engine is now consumed from openbureau-core instead of this repo's own
copy. cms/core = openbureau-core (subtree of its main). docker-compose builds the
cms image from ./core (core's own Dockerfile → core/admin + core/api), and a new
cms/openbureau.config.js (CMS_CONFIG) tells the generic engine this site's content
model + plugins (dialog). Old vendored cms/api + cms/admin removed.

Behaviour-preserving: core's admin/src is byte-identical to the old one, and core's
API was live-verified byte-identical against the running stack. Schema stays owned
by the stack's migrate service (db/schema.sql incl. dialog tables); DATABASE_URL
left unset so core's plugin migration runner is a no-op here. Update core later with
`git subtree pull --prefix=cms/core <core> main --squash`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cms/docker-compose.yml

@@ -146,7 +146,10 @@ services:
   # ════════════════════════════════════════════════════════════════════════
   cms:
     build:
-      context: .
+      # openbureau consumes openbureau-core, vendored via git subtree at cms/core.
+      # Build context = core's root so core's own Dockerfile picks up core/admin +
+      # core/api (the generic engine), not this site's files.
+      context: ./core
       dockerfile: api/Dockerfile
       args:
         # Browser-seitig (Admin-SPA, zur Build-Zeit): öffentliche Supabase-URL.
@@ -169,6 +172,11 @@ services:
       JWT_SECRET: ${JWT_SECRET}
       ADMIN_EMAILS: ${ADMIN_EMAILS:-}
       SITE_DIR: /site
+      # Tells core which content model + plugins this site has (schema-driven engine).
+      # Lives in the mounted repo (/site = repo root). DATABASE_URL is intentionally
+      # unset: the stack's `migrate` service owns the schema (db/schema.sql incl. the
+      # dialog tables), so core's plugin migration runner stays a no-op here.
+      CMS_CONFIG: /site/cms/openbureau.config.js
       PORT: 3000
       GIT_PUBLISH: ${GIT_PUBLISH:-false}
       GIT_REMOTE: ${GIT_REMOTE:-origin}