fix: handle installation events in migrate webhook

[TooAngel]

Summary

The webhook was only handling installation_repositories events, but when a user first installs the app with repos selected, GitHub sends an installation event with action=created instead.

Now handles both:

• installation (action: created) with repositories array
• installation_repositories (action: added) with repositories_added array

Test plan

• Install worlddriven-migrate app on a test repository
• Verify webhook processes the installation event
• Verify migration flow proceeds correctly

[worlddriven]

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🗓️ Merge Date: 2026-01-08 at 02:45:20 UTC (today)
📅 Started: 2026-01-06 at 08:08:41 UTC
⚡ Speed Factor: 0.18 (82% faster due to reviews)
✅ Positive votes: 315/383 contribution weight (coefficient: 0.82)
📈 Base Merge Time: 10 days → Current: 2 days

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📋 Recent Activity

• 2026-01-06, 07:43:45 - Pull request opened
• 2026-01-06, 07:43:47 - Pull request opened
• 2026-01-06, 08:08:49 - Branch synchronized (merge timer reset)
• 2026-01-06, 08:08:50 - Branch synchronized (merge timer reset)
• 2026-01-08, 02:51:45 - Pull request merged by worlddriven ✅

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This comment is automatically updated by worlddriven