feat: make amount of items sent on email configurable via occ

[cristianscheid]

• Resolves: #

Summary

Context

• activity emails are sent to users via the OCA\Activity\BackgroundJob\EmailNotification background job
• the background job calls MailQueueHandler::sendEmails()
• MailQueueHandler::ENTRY_LIMIT defines how many activities are fully shown in the email body, anything beyond that is truncated, as shown below:

Changes introduced by this PR

• adds an admin-configurable setting to control the max number of activities fully shown in notification emails:

occ config:app:set activity mail_max_items --value=500

• falls back to 200 (original value) when no configuration is set
• adds MailQueueHandler::MAIL_MAX_ITEMS_CAP as a safeguard against excessively large values

TODO

• merge documentation PR after this one: feat: add section on activity app admin configuration to include mail_max_items config documentation#15202

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

[cristianscheid]

Please note that psalm CI error is unrelated.

[miaulalala]

Can you please add some tests?