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>

packages/server/src/immich_watcher_server/api/template_configs.py

@@ -69,12 +69,22 @@ async def list_configs(
     user: User = Depends(get_current_user),
     session: AsyncSession = Depends(get_session),
 ):
+    from sqlalchemy import or_
     result = await session.exec(
-        select(TemplateConfig).where(TemplateConfig.user_id == user.id)
+        select(TemplateConfig).where(
+            or_(TemplateConfig.user_id == user.id, TemplateConfig.user_id == 0)
+        )
     )
     return [_response(c) for c in result.all()]
 
 
+@router.get("/variables")
+async def get_template_variables():
+    """Get the variable reference for all template slots."""
+    from .template_vars import TEMPLATE_VARIABLES
+    return TEMPLATE_VARIABLES
+
+
 @router.post("", status_code=status.HTTP_201_CREATED)
 async def create_config(
     body: TemplateConfigCreate,