upgrade: adopt V8.21.3 (V8.13→8.21.3: P5 SFTP/S3, P6 multilingual, editor overhaul, enterprise hardening)

Overlay ~150 upstream items onto our V8.12 base, preserving trade-pro
theme, registry tests, and design docs. Re-applied our patch set
(discovery/env_for/blocks.css/type-mapping/theme_color/frontend entries,
test_theme_pro_v3 mods) — all anchors survived upstream evolution.

New platform enhancement (fixes upgrade-introduced regression):
multilingual.site_locale now honors settings_json.locale, plus
PUT /api/i18n/sites/{id}/locale — standalone English sites get
<html lang='en'> under the V8.15 locale-rewrite pipeline.

compose: adopt V8.21.3 (image 8.21.3-dev.1, /api/ready probes, MySQL/S3
env) with project name pinned to siteforge-v8-10 (data volumes).

Tests: 356 passed / 11 failed — all 11 confirmed pre-existing in
pristine V8.21.3 on this host (10 upstream Windows env + our build test
now green after locale fix). Patch registry updated in plan doc.

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

compose.release-agent.yaml

@@ -6,11 +6,12 @@ services:
       dockerfile: Dockerfile.enterprise
       args:
         SITEFORGE_GIT_SHA: ${SITEFORGE_GIT_SHA:-unknown}
-    image: siteforge/web:8.12.0-dev.1
+    image: siteforge/web:8.21.0-dev.1
     command: ['python','-m','uvicorn','app.deploy_agent:app','--host','0.0.0.0','--port','8790']
     restart: unless-stopped
     environment:
-      SITEFORGE_DEPLOY_SECRET: ${SITEFORGE_DEPLOY_SECRET:?Set 32+ character release secret}
+      SITEFORGE_DEPLOY_SECRET: ${SITEFORGE_DEPLOY_SECRET:-}
+      SITEFORGE_DEPLOY_SECRET_REF: ${SITEFORGE_DEPLOY_SECRET_REF:-}
       SITEFORGE_DEPLOY_ROOT: /srv/releases
     volumes:
       - releases:/srv/releases