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>

scripts/verify_release.py

@@ -6,7 +6,7 @@ import sys
 ROOT=Path(__file__).resolve().parent.parent
 sys.path.insert(0,str(ROOT))
 from app.version import VERSION
-assert VERSION=='8.12.0-dev.1'
+assert VERSION=='8.21.3-dev.1'
 api=(ROOT/'app'/'main.py').read_text(encoding='utf-8')
 assert 'return builder.build(' not in api, 'A synchronous request-thread build has reappeared'
 assert "@app.post('/api/sites/{site_id}/builds', status_code=202)" in api