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>

README.md

@@ -334,15 +334,29 @@ data:
 
 Send notifications to Telegram. Supports multiple formats:
 
-- **Text message** - When `urls` is empty or not provided
-- **Single document** - When `urls` contains one document (default type)
-- **Single photo** - When `urls` contains one photo (`type: photo`)
-- **Single video** - When `urls` contains one video (`type: video`)
-- **Media group** - When `urls` contains multiple photos/videos (documents are sent separately)
+- **Text message** - When `assets` is empty or not provided
+- **Single document** - When `assets` contains one document (default type)
+- **Single photo** - When `assets` contains one photo (`type: photo`)
+- **Single video** - When `assets` contains one video (`type: video`)
+- **Media group** - When `assets` contains multiple photos/videos (documents are sent separately)
 
 The service downloads media from Immich and uploads it to Telegram, bypassing any CORS restrictions. Large lists of photos and videos are automatically split into multiple media groups based on the `max_group_size` parameter (default: 10 items per group). Documents cannot be grouped and are sent individually.
 
-**File ID Caching:** When media is uploaded to Telegram, the service caches the returned `file_id`. Subsequent sends of the same media will use the cached `file_id` instead of re-uploading, significantly improving performance. The cache TTL is configurable in hub options (default: 48 hours, range: 1-168 hours). The cache is persistent across Home Assistant restarts and is stored per album.
+**File ID Caching:** When media is uploaded to Telegram, the service caches the returned `file_id`. Subsequent sends of the same media will use the cached `file_id` instead of re-uploading, significantly improving performance. The cache TTL is configurable in hub options (default: 48 hours, range: 1-168 hours). The cache is persistent across Home Assistant restarts and is shared across all albums in the hub.
+
+**Dual Cache System:** The integration maintains two separate caches for optimal performance:
+
+- **Asset ID Cache** - For Immich assets with extractable asset IDs (UUIDs). The same asset accessed via different URL types (thumbnail, original, video playback, share links) shares the same cache entry.
+- **URL Cache** - For non-Immich URLs or URLs without extractable asset IDs. Also used when a custom `cache_key` is provided.
+
+**Smart Cache Keys:** The service automatically extracts asset IDs from Immich URLs. Supported URL patterns:
+
+- `/api/assets/{asset_id}/original`
+- `/api/assets/{asset_id}/thumbnail`
+- `/api/assets/{asset_id}/video/playback`
+- `/share/{key}/photos/{asset_id}`
+
+You can provide a custom `cache_key` per asset to override this behavior (stored in URL cache).
 
 **Examples:**
 
@@ -366,7 +380,7 @@ target:
   entity_id: sensor.album_name_asset_limit
 data:
   chat_id: "-1001234567890"
-  urls:
+  assets:
     - url: "https://immich.example.com/api/assets/xxx/original?key=yyy"
       content_type: "image/heic"  # Optional: explicit MIME type
   caption: "Original file"
@@ -380,7 +394,7 @@ target:
   entity_id: sensor.album_name_asset_limit
 data:
   chat_id: "-1001234567890"
-  urls:
+  assets:
     - url: "https://immich.example.com/api/assets/xxx/thumbnail?key=yyy"
       type: photo
   caption: "Beautiful sunset!"
@@ -394,7 +408,7 @@ target:
   entity_id: sensor.album_name_asset_limit
 data:
   chat_id: "-1001234567890"
-  urls:
+  assets:
     - url: "https://immich.example.com/api/assets/xxx/thumbnail?key=yyy"
       type: photo
     - url: "https://immich.example.com/api/assets/zzz/video/playback?key=yyy"
@@ -426,17 +440,32 @@ target:
   entity_id: sensor.album_name_asset_limit
 data:
   chat_id: "-1001234567890"
-  urls:
+  assets:
     - url: "https://immich.example.com/api/assets/xxx/thumbnail?key=yyy"
       type: photo
   caption: "Quick notification"
   wait_for_response: false  # Automation continues immediately
 ```
 
+Using custom cache_key (useful when same media has different URLs):
+
+```yaml
+service: immich_album_watcher.send_telegram_notification
+target:
+  entity_id: sensor.album_name_asset_limit
+data:
+  chat_id: "-1001234567890"
+  assets:
+    - url: "https://immich.example.com/api/assets/xxx/thumbnail?key=yyy"
+      type: photo
+      cache_key: "asset_xxx"  # Custom key for caching instead of URL
+  caption: "Photo with custom cache key"
+```
+
 | Field | Description | Required |
 |-------|-------------|----------|
 | `chat_id` | Telegram chat ID to send to | Yes |
-| `urls` | List of media items with `url`, optional `type` (document/photo/video, default: document), and optional `content_type` (MIME type, e.g., `image/jpeg`). Empty for text message. Photos and videos can be grouped; documents are sent separately. | No |
+| `assets` | List of media items with `url`, optional `type` (document/photo/video, default: document), optional `content_type` (MIME type, e.g., `image/jpeg`), and optional `cache_key` (custom key for caching). Empty for text message. Photos and videos can be grouped; documents are sent separately. | No |
 | `bot_token` | Telegram bot token (uses configured token if not provided) | No |
 | `caption` | For media: caption applied to first item. For text: the message text. Supports HTML formatting by default. | No |
 | `reply_to_message_id` | Message ID to reply to | No |