Add Mongo-backed Email Templates with admin UI editor

[krh372]

Problem

Email templates (currently just password-reset and email-verification HTML/subject) are hardcoded as @ElementDefaultAttribute defaults on PasswordResetService/EmailVerificationService. Overriding them requires redeploying code or navigating the element-attribute resolution chain — and a custom Element's own @ElementDefaultAttribute re-declaration for the same key has no effect on the core element that actually reads it, since Tier-2 attribute defaults are scoped per-Element. There's no UI for managing this today.

Goal

Decouple email templates from the attribute system. Introduce a first-class, Mongo-backed EmailTemplate entity addressed by a key string:

• Core templates (password reset, email verification) use reserved, well-known keys and are auto-seeded.
• Custom Elements can mint their own keys and self-register templates via an idempotent getOrCreate call, so their templates show up in the same admin UI with no core platform changes.
• Add an "Email Templates" page under the dashboard's "Other" sidebar category to list/edit these by key.

Scope

• sdk-model: EmailTemplate model + create/update request DTOs
• sdk-dao / mongo-dao / mongo-guice: EmailTemplateDao + Mongo implementation, unique index on key
• sdk-service / service / service-guice: EmailTemplateService (standalone interface, Superuser + Unscoped access), reserved-key validation, getOrCreateEmailTemplate
• rest-api: /email_template CRUD resource
• Migrate AbstractPasswordResetService/AbstractEmailVerificationService off @ElementDefaultAttribute onto the new service
• Admin UI: new EmailTemplates.tsx page, sidebar entry, routing