feat: Add confirmation email for form respondents

[dtretyakov]

Implements issue #525 - Send confirmation emails to form respondents after submission.

Features:

• Add confirmation email settings (enabled, subject, body) to Form entity.
• Implement email sending with placeholder replacement:
  • {formTitle}, {formDescription}
  • Field placeholders based on name or label (e.g. {name}, {email}).
• Allow form creators to explicitly select the recipient email field when multiple email fields are present.
• Add UI in Settings sidebar to configure confirmation emails and recipient selection.
• Replace basic email validation with Nextcloud's internal IEmailValidator.
• Integration with activity notifications and background jobs for file syncing.

Technical changes:

• Database migration for new Form properties.
• Enhanced FormsService with email sending logic and validation.
• Extensive unit and integration tests covering the new functionality.
• Updated API documentation and OpenAPI spec.

[codecov]

Codecov Report

❌ Patch coverage is 73.98374% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/Migration/Version050301Date20260413233000.php	0.00%	25 Missing ⚠️
lib/Service/FormsService.php	92.39%	7 Missing ⚠️

📢 Thoughts on this report? Let us know!

[dtretyakov]

@Chartman123 I also tried to streamline UX for the feature.

Now the form looks like that:

[Chartman123]

@dtretyakov looks good :)

I'll ping the design specialists to see what they think about it :)

[dtretyakov]

OCP\Mail\IEmailValidator breaks backward compatibility for this PR. The interface is not available in nextcloud/ocp on stable31.

For app code here, IMailer::validateMailAddress() is the safer choice: it is already available across the supported server branches and covers the same validation use case without introducing a branch-specific compatibility break.

[Chartman123]

No problem, just ignore this. For the next release we will no longer support NC31