feat: email the new owner when their ownership transfer completes

[DNR500]

Summary

Issue#1769 - sends the new owner a confirmation email once an ownership transfer completes.

What this does

• When someone accepts an ownership nomination, they now get an email confirming it's done
• The email explains what they can do now as owner (manage members, change settings, invite teachers, transfer ownership on, etc.) and links to the Help Center
• No email is sent if the nominee declines instead

Why

Gives the new owner a clear record that the transfer went through, rather than them just finding out by noticing they now have owner access.

[Copilot]

Copilot review overview

🟡 Changes recommended

The email may be sent before roles are transferred and contains hardcoded production links.

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Review effort: Lite
Findings: 1

Open (1)

• Ownership email sent before role transfer completes · New

What changed in this PR

Adds a confirmation email for nominees after an ownership transfer completes.

Changes:

• Adds completion-email delivery and content.
• Adds model and mailer specs plus a preview.
• Includes owner guidance and Help Center links.

File	Summary
spec/​models/​ownership_transfer_spec.rb	Tests completion callback behavior.
spec/​mailers/​school_ownership_mailer_spec.rb	Tests recipients and email content.
spec/​mailers/​previews/​school_ownership_mailer_preview.rb	Adds a completion-email preview.
app/​views/​school_ownership_mailer/​complete_ownership_transfer.text.erb	Defines the confirmation email; links require environment-specific hosts.
app/​models/​ownership_transfer.rb	Adds completion-email delivery; role transfer must occur before sending.
app/​mailers/​school_ownership_mailer.rb	Builds the new-owner confirmation email.

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[github-actions]

Test coverage

93.69% line coverage reported by SimpleCov.
Run: https://github.com/RaspberryPiFoundation/editor-api/actions/runs/35879375810

[zetter-rpf]

Nice one