[#13635] Add more editable templates into custom-email-template feature

[WeeJean]

Part of #13635

Description

This PR completes the end-to-end integration of the Configurable Email Template System. Building on the infrastructure from #3, it migrates all remaining 21 system email templates into the ConfigurableEmailTemplate registry.

By moving away from hardcoded Java constants to a database-first approach, we now allow administrators to safely override any system email at runtime via the Admin UI.

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Key Changes

Backend & Migration

• Full Migration: 21 new templates registered with their
  required placeholder sets, automatically enforced by the existing validation guard.
• Externalized HTML: Created 9 new standalone HTML templates in src/main/resources/, replacing previously inline-only Java strings.
• Refactoring & Cleanup: Deleted 7 orphaned string constants and 4 helper methods, replacing imperative logic with a cleaner declarative registry.
• Instructor Copy Fix: Restored the [Email Copy] subject prefix for co-owner notifications, which was previously bypassed by the new setter.

Infrastructure & Build Fixes

• Codegen Sync: Added EMAIL_TEMPLATE and EMAIL_TEMPLATES to the ResourceEndpoints Java enum. This fixes the CI build failure where api-const.ts was being regenerated without these keys.
• Integration Test Fix: Implemented dynamic date anchoring for awaiting.session test data (+30 days) to prevent regressions caused by sessions falling into the past during execution.

Frontend & Linting

• UI Polish: Updated Jest snapshots for the AdminEmailTemplatesPage and resolved all ESLint violations regarding import ordering and template string literals.

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Migrated Templates (22 total)

• NEW_INSTRUCTOR_ACCOUNT_WELCOME (PR3): Welcome email for new instructor accounts.
• NEW_ACCOUNT_REQUEST_ACKNOWLEDGEMENT: Sent to instructor after submitting an account request.
• NEW_ACCOUNT_REQUEST_ADMIN_ALERT: Sent to admin when a new account request arrives.
• STUDENT_COURSE_JOIN: Student invitation to join a course.
• INSTRUCTOR_COURSE_JOIN: Instructor invitation to join a course.
• STUDENT_COURSE_REJOIN_AFTER_GOOGLE_ID_RESET: Student re-join link after Google ID reset.
• INSTRUCTOR_COURSE_REJOIN_AFTER_GOOGLE_ID_RESET: Instructor re-join link after Google ID reset.
• USER_COURSE_REGISTER: Confirmation when a user registers for a course.
• STUDENT_EMAIL_CHANGED: Course summary sent when a student's email is updated.
• STUDENT_COURSE_LINKS_REGENERATED: New access links sent to student after key regeneration.
• INSTRUCTOR_COURSE_LINKS_REGENERATED: New access links sent to instructor after key regeneration.
• LOGIN: Magic-link login email.
• FEEDBACK_OPENED: Notifies participants when a session opens.
• FEEDBACK_OPENING_SOON: Reminder sent before a session opens.
• FEEDBACK_CLOSING_SOON: Reminder sent before a session closes.
• FEEDBACK_CLOSED: Notifies co-owners when a session closes.
• FEEDBACK_SESSION_REMINDER: Manual reminder blast to incomplete respondents.
• FEEDBACK_PUBLISHED: Notifies participants when results are published.
• FEEDBACK_UNPUBLISHED: Notifies participants when results are unpublished.
• DEADLINE_EXTENSION_GRANTED: Notifies student of a new deadline extension.
• DEADLINE_EXTENSION_UPDATED: Notifies student of a change to their extension.
• DEADLINE_EXTENSION_REVOKED: Notifies student that their extension was removed.

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Not Migrated (3)

• ACCOUNT_REQUEST_REJECTION: Excluded because the subject and body are entirely admin-authored free-form text at call time; there is no fixed template to configure.
• SESSION_LINKS_RECOVERY: Excluded because the body is assembled procedurally by iterating over a variable number of courses and sessions; no static template structure exists to override.
• SEVERE_LOGS_COMPILATION: Excluded as an internal ops-only email; the body is machine-generated log lines and the subject embeds the live app version.