EmailAdapter._seen_uids accumulates every IMAP UID ever seen but never removes any. A long-running gateway processing a high-volume inbox would leak memory indefinitely — thousands of integers per day.

IMAP UIDs are monotonically increasing integers, so old UIDs are safe to drop: new messages always have higher UIDs, and the IMAP UNSEEN flag already prevents re-delivery regardless of our local tracking.

Fix adds _trim_seen_uids() which keeps only the most recent 1000 UIDs (half of the 2000-entry cap) when the set grows too large. Called automatically during connect() and after each fetch cycle.

What does this PR do?

Related Issue

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• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

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