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/migrate_sqlite_to_postgres.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+"""Maintenance-only SQLite to clean PostgreSQL database migration."""
+from __future__ import annotations
+# Allow direct `python scripts/name.py` from a fresh source checkout.
+import sys as _sys
+from pathlib import Path as _Path
+_sys.path.insert(0, str(_Path(__file__).resolve().parents[1]))
+import argparse,json,sys
+from app.sqlite_pg_transfer import migrate
+from app.pg_disaster_recovery import RecoveryError
+
+def main(argv=None):
+    ap=argparse.ArgumentParser(description='Offline SQLite -> EMPTY PostgreSQL data migration')
+    ap.add_argument('--source',required=True,help='Existing SQLite file. Never modified.')
+    ap.add_argument('--confirmed-offline',action='store_true')
+    ap.add_argument('--report',help='Output machine-readable verification summary')
+    args=ap.parse_args(argv)
+    try:
+        result=migrate(args.source,confirmed_offline=args.confirmed_offline)
+        summary=json.dumps(result,ensure_ascii=False,indent=2)
+        if args.report:
+            from pathlib import Path
+            out=Path(args.report).resolve()
+            if out.exists():raise RecoveryError('Refuse overwriting prior validation report')
+            out.parent.mkdir(parents=True,exist_ok=True);out.write_text(summary,encoding='utf-8')
+        print(summary);return 0
+    except (RecoveryError,OSError,ValueError) as ex:
+        print('MIGRATION FAILED: '+str(ex),file=sys.stderr);return 1
+
+if __name__=='__main__':raise SystemExit(main())