refactor(settings): remove global appearance settings — tenant is sole source

Removes the global app_theme, app_theme_user and app_primary_color settings
from the admin/settings area and the underlying service/cache/API/i18n/docs
layers. The tenant columns (tenants.primary_color, default_theme,
allow_user_theme) become the single source of truth for appearance.
Branding helpers are tenant-only and fall back to a hardcoded 'light' theme
with no brand color when no tenant context is available (login, error,
pre-session pages). The APP_THEME env var is removed.

Scope:
- UI: Appearance tab gone from /admin/settings; tenant edit form drops the
  global-fallback color preview.
- Service: SettingsAppGateway no longer exposes setting-backed accessors
  for theme/user-theme/primary-color. Theme catalog utilities (listThemes,
  isAllowedTheme, normalizeTheme, resolveDefaultTheme) stay and are used
  across Tenant / Auth / User flows; resolveDefaultTheme now hardcodes
  'light' with a catalog fallback (no global/env lookup).
- AdminSettingsService: buildPageData, sanitization, audit snapshot, apply
  step and cache payload are all stripped of the three keys. Dead helper
  applyAppPrimaryColor + normalizePrimaryColor removed.
- Cache: SettingCacheService HOT_PATH_KEYS drops the three keys.
- API: /settings (GET/PUT) and /settings/public (GET) no longer expose
  appearance fields; OpenAPI schemas pruned accordingly.
- Audit redaction list shortened.
- DB: db/init/init.sql seed rows removed. No migration for existing
  installs — legacy rows stay inert.
- i18n + docs: setting.app_theme, setting.app_theme_user,
  setting.app_primary_color removed from de+en locales; reference and
  explanation docs updated.
- Tests: covering tests for the removed methods pruned; ThemeResolutionTest
  rewritten for tenant-only semantics. One unrelated PHP-CS-Fixer issue in
  UserRegistrar.php cleaned up to keep QG-006 green.

Workflow: .agents/runs/SETTINGS-APPEARANCE-TENANT-ONLY/ (gitignored).
Gates: QG-001..003, QG-006, QG-008 all pass (1985 tests, 28764 assertions).
Reviews: code, security, acceptance all pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Service/Settings/SettingCacheServiceTest.php

@@ -45,16 +45,13 @@ class SettingCacheServiceTest extends TestCase
         $file = $this->tempFilePath();
 
         $metadataGateway = $this->createMock(SettingsMetadataGateway::class);
-        $metadataGateway->expects($this->exactly(6))
+        $metadataGateway->expects($this->exactly(3))
             ->method('getValue')
             ->willReturnCallback(static function (string $key): ?string {
                 return match ($key) {
                     SettingKeys::APP_TITLE_KEY => 'Demo App',
                     SettingKeys::APP_LOCALE_KEY => 'de',
-                    SettingKeys::APP_THEME_KEY => 'dark',
-                    SettingKeys::APP_THEME_USER_KEY => '1',
                     SettingKeys::APP_REGISTRATION_KEY => '0',
-                    SettingKeys::APP_PRIMARY_COLOR_KEY => '#112233',
                     default => null,
                 };
             });
@@ -66,7 +63,7 @@ class SettingCacheServiceTest extends TestCase
         /** @var array<string, string> $cache */
         $cache = include $file;
         $this->assertSame('de', $cache[SettingKeys::APP_LOCALE_KEY] ?? null);
-        $this->assertSame('#112233', $cache[SettingKeys::APP_PRIMARY_COLOR_KEY] ?? null);
+        $this->assertSame('0', $cache[SettingKeys::APP_REGISTRATION_KEY] ?? null);
     }
 
     private function tempFilePath(): string