Close every reply with an invitation to contact support

[brocla]

Summary

Every reply to a sender now ends with:

Questions, problems, or suggestions? Your feedback is welcome at <support_address>.

• Configured, not hardcoded. The address comes from a new support_address key in config/email.yaml, which keeps the deployment's domain out of the public code. Empty or absent leaves the line off, so replies are unchanged until it's set. An address with no @ fails at load, matching alert_recipient.
• Added in one place. It goes in deliver, after the attach-or-link branch, so it closes attachment and Sheets-link replies, for successful and failed jobs, and comes after the links.
• Not added to operator alerts, which go out through SendAlert.
• Set off by a blank line, not the -- signature separator, because many mail clients dim or fold what follows it.

Test plan

• New internal/mailgun/support_test.go covers:
  • attachment reply ends with the line
  • link reply puts it after the link
  • failed-job reply includes it
  • unconfigured reply is exactly the same as before
  • SendAlert doesn't include it
  • the footer helper and config parsing (trims spaces, off when empty, rejects a value with no @)
• Mutation check: removing the call in deliver makes the three delivery tests fail
• go test ./..., go vet ./..., gofmt -l clean
• After deploy: send one real submission and confirm the line in Gmail

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