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>

app/worker_health.py

@@ -1,18 +1,35 @@
 """Docker worker liveness probe: no localhost Web dependency."""
 import os
 import time
+import socket
+import re
 from pathlib import Path
 from . import db
 
-def healthy(path=None, max_age=None):
-    file=Path(path) if path is not None else db.DATA/'worker-heartbeat'
+def heartbeat_path():
+    instance=os.environ.get('SITEFORGE_WORKER_INSTANCE', '')
+    if not instance and db.POSTGRES:
+        instance=socket.gethostname()
+    if instance:
+        if not re.fullmatch(r'[A-Za-z0-9._-]{1,120}',instance):
+            raise ValueError('Invalid worker instance identifier')
+        return db.DATA/'worker-heartbeats'/instance
+    return db.DATA/'worker-heartbeat'
+
+def healthy(path=None, max_age=None, check_store=False):
+    file=Path(path) if path is not None else heartbeat_path()
     limit=float(max_age if max_age is not None else os.getenv('SITEFORGE_WORKER_HEALTH_MAX_AGE','45'))
     if limit <= 0:return False
     try:
         age=time.time()-file.stat().st_mtime
-        return -5 <= age < limit
-    except OSError:
+        fresh=-5 <= age < limit
+        if not fresh:return False
+        if check_store:
+            from .healthchecks import readiness
+            return readiness()
+        return True
+    except (OSError,ValueError):
         return False
 
 if __name__=='__main__':
-    raise SystemExit(0 if healthy() else 1)
+    raise SystemExit(0 if healthy(check_store=True) else 1)