Add Dev Auth Module with Login/Logout

[cameronhotchkies]

Implement pluggable auth with login/logout flow and session management. Ship with dev-mode fake auth that works without external OAuth — user picker UI for local development.

OSS Note: Must work without internal SSO. AuthModule is swappable by enterprise deployers.

✅ Acceptance Criteria

• Visiting /login shows user picker with test users
• Selecting a user → redirects to app as logged-in user
• Refreshing page preserves logged-in state (session cookie)
• Clicking logout → returns to logged-out state, session cleared
• First-time login creates user in database with generated id
• Returning login finds existing user by externalId

Internal Coding

A2

• adapters/misk/AuthModule.kt — swappable Guice module
• Login WebAction with user picker UI
• Session cookie handling
• Logout endpoint