test: cover auth template flows

[newmattock]

Summary

• Adds focused Vitest coverage for auth-template JSON responses, password policy checks, and stable password hashing.
• Adds mocked email signup coverage for missing fields, policy rejection, duplicate users, user creation failure, and successful user/session creation.
• Adds mocked email login coverage for missing fields, invalid credentials, session creation failure, and successful session creation.

Refs #71
/claim #71

Tests

• pnpm exec vitest --run templates/auth/src/utils.test.ts templates/auth/src/email/index.test.ts -> 19 tests passed
• pnpm exec prettier --check templates/auth/src/utils.test.ts templates/auth/src/email/index.test.ts -> passed
• git diff --check -> passed
• pnpm exec vitest --run -> existing src/rls/index.test.ts baseline failures remain (4 RLS policy WHERE-clause expectations fail); the new auth-template tests pass in that full run.