[Feature]: Add Signal-based SMS notification support to WSD using signal-cli

[mophawka]

💡 Feature Description
Add support for sending WSD notifications via Signal using the signal-cli utility.
This would allow admins to securely push alerts and status updates to yourself(Admin), family members, friends, or users who don’t use Discord or other messaging platforms.

The implementation would follow an approach similar to what is outlined in this blog: [Signal Messaging via CLI/API] and leverage the open-source project [AsamK/signal-cli]

📈 Motivation and Context
Not every user or household member wants or needs to join a Discord server or install a separate dashboard app. Signal offers:
End-to-end encrypted messaging
Cross-platform support
Familiar UX for many users
Strong privacy guarantees

By integrating Signal notifications into WSD, we create a secure, simple alternative for:
Downtime alerts
New release notifications
Device login warnings
Watchlist updates
This is especially useful in a family media server context.

🛠️ Suggested Implementation (if known)
Use signal-cli in headless or daemon mode
Add config section in WSD settings:
Signal sender (number or UUID as supported)
List of recipients (phone numbers or UUIDs)
Optional message templates per notification type

How to send a message (basic):
signal-cli -u +1234567890 send -m "New episode of The Bear is available!" +1987654321
Signal-cli setup options:

Use an existing phone number or register a headless UUID
Ref: signal-cli/discussions/1485

Notification types:
Media availability alerts
System/server errors
Device login notifications
Personalized updates per user
Provide a test button in the UI to verify messaging setup

🖼️ Visuals or References (optional)
Blog reference: https://blog.aawadia.dev/2023/04/24/signal-api/

GitHub repo: https://github.com/AsamK/signal-cli

Example of a WSD -> Signal message:
🎬 New Watch Alert:
“Oppenheimer” is now available on the server!

• Added 1h ago

🔗 Related Issues or Discussions
signal-cli headless mode setup: AsamK/signal-cli#1485

🧩 Additional Notes
Include retry/failover logic for undelivered messages
Ensure CLI logs don’t expose phone numbers in plaintext