feat: mark glossary templates as multilingual — support 11 target languages

Templates enriched by enrich_glossary_templates.py already contain
translations for de, es, it, pt, nl, ru, ja, ko, zh, ar, fa (including
Persian). But they were labeled FR→EN, causing incorrect filtering and
warnings when translating to other languages.

Changes:
- index.json: set target_lang='multi' for all 8 templates
- GlossarySelector: treat target_language='multi' as compatible with
  any target language (no false warnings, auto-select works)
- GlossarySelector: display '🌐 MULTILINGUE' badge instead of EN flag
- glossary_routes: default target_language to 'multi' instead of 'en'
- Migration: detect existing multilingual glossaries in DB (5+ keys in
  translations JSON) and set their target_language to 'multi'

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

routes/glossary_routes.py

@@ -57,7 +57,7 @@ def _format_glossary(glossary: Glossary) -> dict:
         "id": glossary.id,
         "name": glossary.name,
         "source_language": glossary.source_language,
-        "target_language": getattr(glossary, "target_language", "en") or "en",
+        "target_language": getattr(glossary, "target_language", "multi") or "multi",
         "terms": [_format_term(t) for t in glossary.terms] if glossary.terms else [],
         "created_at": glossary.created_at.isoformat() if glossary.created_at else None,
         "updated_at": glossary.updated_at.isoformat() if glossary.updated_at else None,