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>

tests/test_v8_workflows.py

@@ -1,3 +1,4 @@
+from app.version import VERSION
 import json
 import pytest
 from fastapi import HTTPException
@@ -145,7 +146,7 @@ def test_v8_api_authenticated_and_browser_page(env):
         assert client.get(f'/api/factory/sites/{sid}/staging').json()['items'][0]['state']=='committed'
         snap=client.post(f'/api/factory/sites/{sid}/whole-site-templates',json={'name':'Test'})
         assert snap.status_code==200,snap.text
-        assert client.get('/api/health').json()['version']=='8.12.0-dev.1'
+        assert client.get('/api/health').json()['version']==VERSION
 
 
 def test_wxr_preserves_post_type(env):