Replace TTL with thumbhash-based cache validation and add Telegram video size limits

- Asset cache now validates entries by comparing stored thumbhash with current
  Immich thumbhash instead of using TTL expiration. This makes cache invalidation
  precise (only when content actually changes) and eliminates unnecessary re-uploads.
  URL-based cache retains TTL for non-Immich URLs.
- Add TELEGRAM_MAX_VIDEO_SIZE (50 MB) check to skip oversized videos in both
  single-video and media-group paths, preventing entire groups from failing.
- Split media groups into sub-groups by cumulative upload size to ensure each
  sendMediaGroup request stays under Telegram's 50 MB upload limit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

custom_components/immich_album_watcher/translations/en.json

@@ -205,9 +205,9 @@
                     "name": "Chat ID",
                     "description": "Telegram chat ID to send to."
                 },
-                "urls": {
-                    "name": "URLs",
-                    "description": "List of media URLs with optional type (document/photo/video, default: document) and optional content_type (MIME type). If empty, sends a text message. Photos and videos can be grouped; documents are sent separately."
+                "assets": {
+                    "name": "Assets",
+                    "description": "List of media assets with 'url', optional 'type' (document/photo/video, default: document), optional 'content_type' (MIME type), and optional 'cache_key' (custom key for caching instead of URL). If empty, sends a text message. Photos and videos can be grouped; documents are sent separately."
                 },
                 "caption": {
                     "name": "Caption",