🔧 Mail call sites: drop hand-built appName/appContact, brand the subjects

[PierreBrisorgueil]

What

• Remove appName and appContact from the params at the 14 sendMail sites, since the mailer now injects them.
• Switch the 8 subjects that embed the app title to getBrand().name:
  • billing.email.js: 5 subjects
  • billing.referral.service.js:174
  • invitations.service.js:27
  • organizations.service.js:159
• Subjects built from org.name, and the 4 fixed English subjects, are unchanged.

Acceptance criteria

• No call site reads config.app.title or config.app.contact to build a mail.
• With no brand config, rendered bodies and subjects are unchanged, and the baseline snapshot still passes.
• With mailer.brand.name set, the body and subject of each mail that names the app use the same name.

Tests

• Update billing.init.email-alerts, organizations.membership.addMember.email and organizations.service.welcomeEmail.
• Add a subject assertion with a brand-name override.

Depends on: brand.
Refs: auth.password.controller, auth.signup.service, users.service, invitations.service, organizations(.membership).service, billing.email, billing.referral.service

Needs: #4128

[PierreBrisorgueil]

WIP — session 20261003T215404Z-BFB390C4, branch feat/4131-drop-appname-appcontact, started 2026-10-03T21:54:04Z

[PierreBrisorgueil]

merged via drain (Automerge: absent, --review: false) — PR #4177. Reviewers: Phase 0 Kimi + Sonnet/security, CodeRabbit approved on final head.