The Stage 4 - Post Release Validation workflow was not being triggered after alpha releases completed. This meant that alpha packages were published to the APT repository without the validation step that ensures they can be installed correctly and that Homebridge starts successfully.

Problem

The Stage 4 workflow only listened for completion of:

• "Stage 3 - Promote Release Package to APT Stores"
• "Beta Stage 2 - Build and Release Beta Package"

However, alpha releases use a different workflow: "Alpha Stage 2 - Build and Release Alpha Package", which publishes directly to the alpha APT repository without going through Stage 3.

Solution

This PR adds alpha support to the Stage 4 validation workflow by:

1. Adding alpha workflow trigger: Added "Alpha Stage 2 - Build and Release Alpha Package" to the workflow_run.workflows list so Stage 4 runs after alpha releases complete
2. Adding alpha channel detection: Added logic to detect when the alpha workflow completes and automatically set the validation channel to "alpha"
3. Adding manual alpha option: Added "alpha" as an option for manual workflow dispatch testing

Changes

• Stage 4 now automatically runs after successful alpha releases
• Alpha packages are validated against the alpha APT channel (https://repo.homebridge.io alpha main)
• Manual testing of alpha packages is now possible via workflow dispatch
• All existing stable and beta validation behavior remains unchanged

The implementation follows the existing pattern used for beta releases, ensuring consistency across all release types.

Fixes #177.

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