Add optional Gotify notifications

[deejayexe]

Summary

Adds optional Gotify notifications to ImageMaid while preserving the existing Discord notification functionality.

Changes

• Add optional Gotify notifications, disabled by default.
• Support configurable Gotify server URL, application token, and message priority.
• Send notifications when an ImageMaid run starts, completes, or encounters a handled error.
• Include run information and results in Markdown-formatted messages.
• Support notifications for immediate and scheduled runs.
• Handle Gotify delivery failures without interrupting ImageMaid.
• Document all new environment variables in README.md.
• Add unit tests for the Gotify notifier.

Configuration

Enable Gotify by setting:

GOTIFY_ENABLED=True
GOTIFY_URL=https://gotify.example.com
GOTIFY_TOKEN=YOUR_APPLICATION_TOKEN

Additional options:

• GOTIFY_PRIORITY
• GOTIFY_NOTIFY_START
• GOTIFY_NOTIFY_END
• GOTIFY_NOTIFY_ERROR

Testing

• Python 3.11 syntax validation passed.
• All 9 unit tests passed.
• Successfully built the Docker image using the official Dockerfile.
• Tested against a live Plex server in report mode with a read-only Plex filesystem mount.
• Confirmed successful delivery of start and completion notifications to Gotify.
• Verified that git diff --check passes.

Notes

• Existing Discord notification calls remain unchanged.
• Gotify notification failures do not stop ImageMaid execution.
• KeyboardInterrupt cancellation does not currently trigger a Gotify completion notification.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}