fix: embed Nextcloud logo directly into SMTP notifications

[felixschndr]

Summary

This embeds the Nextcloud logo directly into the email (SMTP notification). Before the nextcloud logo would not show up when the instance is not public accessibly:

After:

Same as seerr-team/seerr#3034 (see seerr-team/seerr#3034 (comment))

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[github-actions]

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