feat: module-owned i18n with merge-at-boot translation loading

Each module can now ship its own i18n/ directory with translation files
(default_de.json, default_en.json). At boot time, ModuleI18nLoader
preloads core translations then merges module translations on top via
Reflection into I18n::$strings — the t() helper stays unchanged.

- Add i18nPath property to ModuleManifest
- Add ModuleI18nLoader with Reflection-based preload
- Extract 59 module-exclusive keys from core i18n into module files
  (notifications: 8, bookmarks: 41, addressbook: 10)
- Expand TranslationKeysTest to scan modules/ and validate against
  merged core + module translations
- Add ModuleI18nContractTest for per-module de/en parity and completeness

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/addressbook/module.php

@@ -94,4 +94,5 @@ return [
     ],
 
     'migrations_path' => 'db/updates',
+    'i18n_path' => 'i18n',
 ];