✨ Mailer: resolve templates through config.mailer.templates #4129

What

• Add mailer.templates: {} to development.config.js. It maps a key to a relative .html path.
• Extract render(key, params, { subject } = {}), which returns the html, and export it. sendMail calls it.
• Resolution:
  • The key goes through path.basename.
  • A key in the map uses its path.
  • Any other key falls back to config/templates/<key>.html, which is today's behavior.
• Containment: a configured path must be relative, end in .html and resolve (path.relative) inside the project root. Otherwise throw an error that names the key.

Acceptance criteria

• Callers are unchanged, and every existing key renders the same HTML as today.
• A map entry pointing to config/templates/<project>/x.html wins over the flat file.
• Keys not in the map still resolve from the flat folder, so templates that exist only in a downstream keep working.
• Absolute paths, paths containing .. that escape the root, and non-.html paths are rejected.

Tests

• Resolution order: a map hit, a map miss that falls back, and a key containing a traversal attempt.
• Containment: an absolute path, .., and the wrong extension.
• Baseline render over all 15 shared templates: each template gets fixture params. Assert that every {{variable}} it references is present in its fixture. Snapshot the normalized text; the fragments slice diffs against this snapshot.
• Make the files.readFile mock path-aware.

Depends on: nothing (it can land in parallel with the billing-links fix).
Refs: lib/helpers/mailer/index.js:82-99, lib/helpers/mailer/tests/mailer.unit.tests.js