Add invitation expiration to AccountsInvitation

[dadachi]

Summary

• Add configurable expiration to AccountsInvitation (48h production, ~5min development)
• Return 410 Gone when accessing or accepting expired invitations
• Resend action now resets expiration by touching created_at before re-sending the invite email
• Add expiration notice to invitation email templates (HTML and text)

Test plan

• bin/rails test — 334 tests, 651 assertions, 0 failures
• bin/rubocop — No offenses
• bin/brakeman — No security warnings
• Verify expired invitation returns 410 on show/update in development (~5 min expiry)
• Verify resend resets expiration timer
• Verify invitation email includes expiration notice

🤖 Generated with Claude Code