Update axllent/mailpit Docker tag to v1.31

[renovate]

This PR contains the following updates:

Package	Update	Change
axllent/mailpit (source)	minor	v1.30 → v1.31

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Release Notes

axllent/mailpit (axllent/mailpit)

v1.31

Feature

• Add deep dark theme (#​728)
• Add debounced search refresh for filtered views on new messages

Chore

• Make read/unread message state clearly distinguishable (WCAG AA)
• Add accessible names to icon-only buttons and fix sender tag mismatch
• Update Go dependencies
• Update node dependencies
• Update caniemail test database
• Update GitHub Actions

Fix

• Include all IANA special-use IPv4 ranges in IsInternalIP check
• Prevent SMTP command injection via drainData fragment-boundary bypass
• Validate Send API custom header keys per RFC 5322

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