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/check.sh

@@ -2,7 +2,10 @@
 set -euo pipefail
 cd "$(dirname "${BASH_SOURCE[0]}")/.."
 python -m compileall -q app
-python -m pytest -q
+# Each batch has a hard deadline and failed batches are isolated for diagnosis.
+# Reports are generated outside test output; do not commit logs without review.
+python scripts/regression_v821.py --repeat "${SITEFORGE_REGRESSION_REPEAT:-1}" --timeout "${SITEFORGE_REGRESSION_TIMEOUT:-90}" --report reports/v821-regression
+python scripts/verify_release.py
 if command -v node >/dev/null 2>&1; then
   for f in app/static/*.js; do node --check "$f"; done
 else