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>

requirements-dev.lock

@@ -35,3 +35,16 @@ webencodings==0.5.1
 
 PyYAML==6.0.3
 PyMySQL==1.1.2
+
+# Native P5 publish connectors (SFTP + S3), pinned for CI/Docker.
+boto3==1.43.18
+botocore==1.43.18
+s3transfer==0.18.0
+jmespath==1.1.0
+python-dateutil==2.9.0.post0
+urllib3==2.7.0
+paramiko==4.0.0
+bcrypt==4.3.0
+cryptography==46.0.4
+cffi==2.0.0
+pycparser==3.0