[cuegui] Add email notifications for subscription size/burst changes

[ramonfigueiredo]

Describe the enhancement
We currently do not have a history of changes for Cue subscription values. When subscription size or burst values are changed through the CueCommander GUI, there is no audit trail or notification system to track who made the changes and when.

This creates transparency issues, as Subscriptions are often adjusted without the knowledge of relevant teams (production support, resource management, etc.). Since adding a full change history system would be a significant undertaking, we propose a simpler solution: send an automated email notification each time a subscription's size or burst value is modified through the CueCommander GUI.

The email should include:

• The subscription name, show, and allocation
• What was changed (size or burst)
• The old value and new value
• Who made the change (username)
• When the change was made (timestamp)

Proposed Configuration:

The email recipients should be configurable in cuegui.yaml to allow different organizations to customize who receives these notifications.

For example:

email.subscription_change_cc: "{show}-admin@{domain},resource-team@{domain}"

This would support:
- Template placeholders ({show}, {domain}) for dynamic email addresses
- Static email addresses
- Comma-separated lists for multiple recipients
- Empty string to disable notifications

Example Email:

Subject: Subscription Size Changed: local.general.myshow

A subscription size has been modified.

Subscription: local.general.myshow
Show: myshow
Allocation: local.general

Change Details:
Size: 100.0 - > 150.0

Modified by: jsmith
Timestamp: 2025-10-14 14:30:00

This is an automated notification from CueCommander.

**Version Number**
- https://github.com/AcademySoftwareFoundation/OpenCue/releases/tag/v1.13.8

**Additional context**
- Email notifications should NOT block the subscription change operation - if the email fails to send, the change should still succeed and a warning should be logged
- The submitter does not need to be CC'd on the notification
- This would provide an immediate, actionable audit trail without requiring extensive database changes for full history tracking