fix(gmail-watch): delay history fetch in watch serve

[salmonumbrella]

Summary

• add --fetch-delay to gog gmail watch serve (default 3s, accepts seconds or Go durations)
• delay history fetch after push delivery to avoid Gmail indexing race conditions
• add validation and server coverage tests for fetch delay behavior and context cancellation
• document the new flag in watch docs and README

Closes #379

Verification

• make test
• go test ./internal/cmd -run 'GmailWatchServeCmd|GmailWatchServer_HandlePush_AppliesFetchDelay|GmailWatchServer_HandlePush_FetchDelayCanceledContext'

[steipete]

Landed via temp rebase onto main.

• Gate: make ci
• Land commit: d22eb17
• Merge commit: caf38a3

Thanks @salmonumbrella!