feat: add mailing notifications to calendar delegation

[GVodyanov]

Summary

Needed for nextcloud/calendar#8085

This PR consists of two parts:

1. feat: notify delegatee of new calendar delegations
2. fix: make delegated calendar homes discoverable

TODO

• Requires fix: add proper ACLs for trashbin proxys #60211

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

[ChristophWurst]

#16666 added delegation. Can you update the description to explain what this PR adds on top, and how it can be tested?

[hamza221]

I'm going to extract the fix for #42051 in a seprate PR then we rebase this one

[GVodyanov]

#16666 added delegation. Can you update the description to explain what this PR adds on top, and how it can be tested?

1. Email notifications
2. Calendar home resolution for proxy principals, so that the delegatee can get the delegator's calendars
3. ACL on proxy group principals
4. The trashbin ACL fix that Hamza will now move elsewhere

Correct me if I'm wrong @hamza221

[GVodyanov]

There is a comment from Sebastian nextcloud/calendar#8085 (comment)

[ChristophWurst]

4. The trashbin ACL fix that Hamza will now move elsewhere

Correct me if I'm wrong @hamza221

Seems to be at #60211 and a requirement for this PR.

I have now updated the PR description for you with this information. Correct it if I got anything wrong.

Since this PR now contains a new feature and fixes, I do wonder if the fixes shouldn't be separate and backported to also fix discovery for older releases that already had delegation.

[GVodyanov]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Done in 095b3a7. I resolved the merge conflict hotspots by aligning the two conflicting files with current master-compatible behavior (TrashbinHome proxy-write ACL and files_versions principalBackend registration), and validated with a merge-tree conflict check plus Code Review/CodeQL.

[hamza221]

closed in favor of #60712