docs: update deployment README to reflect auto-deploy

[cfsmp3]

Summary

Updates the deployment README to accurately describe the new auto-deploy workflow.

Changes

• Overview now says "automatic deployment" instead of "manually triggered"
• Deployment section describes the actual auto-deploy flow
• Removed outdated "after migration" note from monitoring section

Purpose

This is a documentation-only change to test that the auto-deploy pipeline works correctly.

After merging, watch GitHub Actions to verify:

1. Frontend and backend images build
2. Deploy job runs automatically
3. VPS receives the new deployment

🤖 Generated with Claude Code

[github-actions]

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