fix: advance Gmail cursor past missing messages

[sangowu]

What changed

• treat Gmail messages.get 404 responses as terminal missing-message conditions
• skip disappeared messages without incrementing failed_messages, allowing the latest historyId to be saved
• keep authentication, rate-limit, server, and network failures retryable so they still hold the cursor
• add regression tests and update the changelog and bug-fix log

Root cause

The fetch loop counted every exception as a failed message. Cursor persistence requires failed_messages == 0, so one message that had disappeared after appearing in Gmail history permanently blocked the incremental history cursor and produced the same warning every 15 minutes.

Validation

• 63 passed in the email application-tracking suite
• 247 passed in the full test suite
• Ruff passed
• frontend JavaScript syntax check passed
• Python compileall passed
• git diff --check passed