Embed app logo in email templates

[jbrooksuk]

Summary

• embed the configured app logo in outgoing emails as an inline CID attachment
• add configurable upload disk, maximum size, and accepted image MIME types
• apply the configured disk consistently to uploads, status-page rendering, and email attachments
• add regression coverage for existing stored logos and upload validation

Configuration

• CACHET_UPLOAD_DISK (default: public)
• CACHET_UPLOAD_MAX_SIZE (default: 1024 KB)
• CACHET_UPLOAD_IMAGE_MIME_TYPES (default: JPEG, PNG, GIF, and WebP)

Existing uploaded logos remain compatible because the disk defaults to the previously hard-coded public disk and the stored path format is unchanged.

Verification

• 1,041 tests passed, 2 todos
• Pint passed
• PHPStan passed with no errors
• frontend build passed
• git diff --check passed

Closes #436

[chatgpt-codex-connector]

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