feat(forms): add submission notification emails with external recipients and PDF attachments

[QuiteBitter]

What this changes

• Adds submission notification mails for new responses using the configured system mail account.
• Lets form owners opt into owner notifications and configure additional external notification recipients per form.
• Lets forms attach a generated PDF copy of the submission to those notification mails.
• Validates and normalizes configured recipient addresses in the API and settings UI, and covers the notification / attachment flow with unit and integration tests.

Related issues

• Send content of response as mail notification to form owner #1233
• Submit forms submissions direct to external email addresses #1810
• Send email with submission-pdf to owner #883

Notes

• Additional recipients are independent of the owner-notification switch, so a form can notify external addresses without mailing the owner.
• The mail body keeps the inline summary focused on short and long text answers; enabling the PDF attachment adds a portable copy of the captured submission data for forwarding and archiving.
• Leaves generated locale catalogs out of the PR because Nextcloud translations are handled through Transifex.

Validation

• Nextcloud 33: unit and integration suites passed.
• Nextcloud 31: unit and integration suites passed.

[Chartman123]

Thanks for your PR! Please understand that we're currently migrating to Vue3 and won't merge new features until this has finished to minimize extra work.

[QuiteBitter]

Thanks for your PR! Please understand that we're currently migrating to Vue3 and won't merge new features until this has finished to minimize extra work.

Okey, I will wait for this to complete :) good luck! After completion, I'll check what's needed to be done on my side.