Replace video_warning with target_type + has_videos/has_photos

Major template system improvements:
- Remove video_warning field from TemplateConfig model
- Add target_type, has_videos, has_photos to template context
- Templates use {% if target_type == "telegram" and has_videos %}
  for conditional Telegram warnings instead of a separate field
- date_format moved from "Telegram" to "Settings" group
- Add target type selector (Telegram/Webhook) in template editor
  to preview how templates render for each target type
- All template slots now use JinjaEditor (not plain <input>)
- Preview endpoint accepts target_type parameter
- Clean up TemplateConfigCreate schema (remove stale fields)

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

packages/server/src/immich_watcher_server/templates/en/assets_added.jinja2

@@ -9,7 +9,7 @@
     {%- if asset.is_favorite %} ❤️{% endif %}
 {%- endfor %}
 {%- endif %}
-{%- if video_warning %}
+{%- if target_type == "telegram" and has_videos %}
 
-{{ video_warning }}
+⚠️ Videos may not be sent due to Telegram's 50 MB file size limit.
 {%- endif %}