Simplify templates to pure Jinja2 + CodeMirror editor + variable reference

Major template system overhaul:
- TemplateConfig simplified from 21 fields to 9: removed all sub-templates
  (asset_image, asset_video, assets_format, people_format, etc.)
  Users write full Jinja2 with {% for %}, {% if %} inline.
- Default EN/RU templates seeded on first startup (user_id=0, system-owned)
  with proper Jinja2 loops over added_assets, people, albums.
- build_full_context() simplified: passes raw data directly to Jinja2
  instead of pre-rendering sub-templates.
- CodeMirror editor for template slots (HTML syntax highlighting,
  line wrapping, dark theme support via oneDark).
- Variable reference API: GET /api/template-configs/variables returns
  per-slot variable descriptions + asset_fields for loop contexts.
- Variable reference modal in UI: click "{{ }} Variables" next to any
  slot to see available variables with Jinja2 syntax examples.
- Route ordering fix: /variables registered before /{config_id}.

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

frontend/package.json

@@ -29,6 +29,11 @@
 		"vite": "^7.3.1"
 	},
 	"dependencies": {
-		"@mdi/js": "^7.4.47"
+		"@codemirror/lang-html": "^6.4.11",
+		"@codemirror/state": "^6.6.0",
+		"@codemirror/theme-one-dark": "^6.1.3",
+		"@codemirror/view": "^6.40.0",
+		"@mdi/js": "^7.4.47",
+		"codemirror": "^6.0.2"
 	}
 }