Track pending assets for delayed processing events

- Add _pending_asset_ids to track assets detected but not yet processed
- Fire events when pending assets become processed (thumbhash available)
- Fixes issue where videos added during transcoding never triggered events
- Add debug logging for change detection and pending asset tracking
- Document external domain feature in README

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -79,6 +79,29 @@ A Home Assistant custom integration that monitors [Immich](https://immich.app/)
 | Telegram Bot Token | Bot token for sending media to Telegram (optional) | - |
 | Telegram Cache TTL | How long to cache uploaded file IDs (hours, 1-168) | 48 |
 
+### External Domain Support
+
+The integration supports connecting to a local Immich server while using an external domain for user-facing URLs. This is useful when:
+
+- Your Home Assistant connects to Immich via local network (e.g., `http://192.168.1.100:2283`)
+- But you want share links and asset URLs to use your public domain (e.g., `https://photos.example.com`)
+
+**How it works:**
+
+1. Configure "External domain" in Immich: **Administration → Settings → Server → External Domain**
+2. The integration automatically fetches this setting on startup
+3. All user-facing URLs (share links, asset URLs in events) use the external domain
+4. API calls and file downloads still use the local connection URL for faster performance
+
+**Example:**
+
+- Server URL (in integration config): `http://192.168.1.100:2283`
+- External Domain (in Immich settings): `https://photos.example.com`
+- Share links in events: `https://photos.example.com/share/...`
+- Telegram downloads: via `http://192.168.1.100:2283` (fast local network)
+
+If no external domain is configured in Immich, all URLs will use the Server URL from the integration configuration.
+
 ## Entities Created (per album)
 
 | Entity Type | Name | Description |