Update README with album date placeholders documentation

Added {album_created} and {album_updated} to:
- Message Template Variables
- Asset Item Template Variables
- Album Item Template Variables (also added {album_id})
- Scheduled Assets Message Template Variables

Also added missing {common_date} and {common_location} to
Scheduled Assets documentation.

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

Common/Immich Album Watcher/README.md

@@ -83,6 +83,8 @@ All message templates support these placeholder variables (use single braces):
 |----------|-------------|
 | `{album_name}` | Name of the album |
 | `{album_url}` | Public URL to view the album (empty if no shared link) |
+| `{album_created}` | Album creation date (formatted with date format) |
+| `{album_updated}` | Album last update date (formatted with date format) |
 | `{added_count}` | Number of assets added |
 | `{removed_count}` | Number of assets removed |
 | `{people}` | Comma-separated list of people detected |
@@ -109,6 +111,8 @@ These variables can be used in the image and video asset templates. Also used fo
 | `{playback_url}` | Video playback URL (for VIDEO assets only) |
 | `{people}` | People detected in this asset |
 | `{album_name}` | Name of the album |
+| `{album_created}` | Album creation date (formatted with date format) |
+| `{album_updated}` | Album last update date (formatted with date format) |
 | `{is_favorite}` | Favorite indicator (using template) if asset is favorite, empty otherwise |
 | `{rating}` | User rating (1-5) or empty if not rated |
 | `{location}` | Formatted location string (using location format template), empty if location data incomplete |
@@ -161,7 +165,10 @@ Sends a summary notification of tracked albums at regular intervals. Album names
 | Variable | Description |
 |----------|-------------|
 | `{album_name}` | Name of the album |
+| `{album_id}` | Album ID |
 | `{album_url}` | Share URL for the album |
+| `{album_created}` | Album creation date (formatted with date format) |
+| `{album_updated}` | Album last update date (formatted with date format) |
 
 ### Testing Periodic Summary
 
@@ -204,8 +211,12 @@ Sends scheduled notifications with existing assets from tracked albums. Uses the
 |----------|-------------|
 | `{album_name}` | Name of the album |
 | `{album_url}` | Share URL for the album |
+| `{album_created}` | Album creation date (empty in combined mode) |
+| `{album_updated}` | Album last update date (empty in combined mode) |
 | `{asset_count}` | Number of assets fetched |
 | `{assets}` | Formatted list of assets (using asset item template) |
+| `{common_date}` | Common date if all assets share same date |
+| `{common_location}` | Common location if all assets share same location |
 
 ### Testing Scheduled Assets