Add separate WhatsApp Business templates for signature request and completion notifications

Describe the bug

LibreSign currently uses the same WhatsApp Business message template for different stages of the signing workflow. Signature requests and signature completion notifications have different purposes and recipients, so each event should use its own dedicated template.

To reproduce

• Enable WhatsApp Business notifications for LibreSign.
• Send a document to a recipient for signature.
• Verify the WhatsApp message received by the signer.
• Complete the signature process.
• Keep the notification for completed signatures enabled for the signer.
• Verify the WhatsApp message received after the signature is completed.
• Observe that the same WhatsApp Business template is currently being reused for both events, even though they represent different notification contexts.

Expected behavior

Add separate WhatsApp Business templates for signature request and completion notifications

To reproduce

1. Enable WhatsApp Business notifications for LibreSign.
2. Send a document to a recipient for signature.
3. Verify the WhatsApp message received by the signer.
4. Complete the signature process.
5. Keep the notification for completed signatures enabled for the signer.
6. Verify the WhatsApp message received after the signature is completed.
7. Observe that the same WhatsApp Business template is currently being reused for both events, even though they represent different notification contexts.

Expected behavior

LibreSign should use two different WhatsApp Business templates:

• Signature request template

  • Sent to the person who must sign the document.
  • Triggered when the document is sent for signature.
  • The message must inform the recipient that a document is waiting for their signature and provide the signing link.

• Signature completed template

  • Sent to the signer after the signature process is successfully completed.
  • It must only be sent when the signer has the corresponding completion notification enabled.
  • The message must inform the signer that the document has been successfully signed.

Screenshots

Additional context

Additional context

To reproduce and validate this issue locally, the TwoFactor Gateway app must be installed and configured in the development environment.

A WhatsApp Business account must also be configured so the notification templates can be tested through the actual integration and both message flows can be validated locally.

When submitting the PR, please include screenshots of the implemented solution showing how both WhatsApp messages are received:

• The message requesting the recipient to sign the document.
• The message confirming that the document was signed successfully.

No files or tests are named. Start by tracing LibreSign's WhatsApp Business notification paths for signature requests and completed signatures, then validate both flows with the TwoFactor Gateway app and a configured WhatsApp Business account. Done means each event uses its own template, completion messages respect the signer's notification setting, and screenshots show both messages.