feat(notification): add interval for new question emails

[AsyncAssassin]

Summary

Adds a configurable interval between email send attempts for new-question / followed-tag notification fan-out.

The interval is controlled by NEW_QUESTION_NOTIFICATION_EMAIL_SEND_INTERVAL_SECONDS.

The default is 0, so the current behavior is preserved unless the environment variable is explicitly set. Empty, invalid, negative, or overflowing values also fall back to 0.

Scope

This is based on the scope discussed in #625 and confirmed by @LinkinStars.

This PR only applies the delay to new-question notification email fan-out. It does not change:

• daily digest behavior
• logged-in user handling
• verification emails
• password reset emails
• SMTP test emails
• the shared EmailService.Send
• UI/admin settings
• migrations

When the interval is enabled, plugin notifications are sent before the throttled email fan-out, so they are not delayed by the configured sleep.

Tests

Added focused tests for:

• environment variable parsing
• delay ordering between email attempts
• disabled/non-email channel handling
• unsubscribe token generation per email attempt
• service-level new-question fan-out behavior with fake service dependencies

Validation:

go test -count=1 ./internal/service/notification/...
go test -race -count=1 ./internal/service/notification/...
go test -count=1 ./internal/...
go test -count=1 ./...
go vet ./internal/service/notification/...
git diff --check