feat: add local auth, proxy auth, health checks, and improved docs

[brian-olson]

Summary

This PR adds several improvements for self-hosted deployments, addressing multiple open issues:

Local Username/Password Authentication (Fixes #116)

• Add LOCAL_AUTH_ENABLED and LOCAL_AUTH_REGISTRATION_ENABLED environment variables
• Add /api/1/auth/local/login endpoint for username/password login
• Add /api/1/auth/local/register endpoint for self-registration (when enabled)
• Add /api/1/auth/change-password endpoint for password changes
• Add /api/1/auth/status endpoint to show available authentication methods

This allows self-hosted instances to work without OAuth configuration - just set LOCAL_AUTH_ENABLED=true.

Proxy Authentication (Also addresses #116)

• Add PROXY_AUTH_ENABLED setting for reverse proxy authentication
• Add ProxyAuthMiddleware to trust X-Auth-* headers from auth proxies
• Supports Authentik, Authelia, Traefik ForwardAuth, and similar solutions
• Headers: X-Auth-User-Email (required), X-Auth-User-Name (optional)

Health Check Endpoints

• Add /health endpoint for basic health checks (load balancers)
• Add /ready endpoint for readiness checks (includes DB connectivity)
• Add /live endpoint for liveness checks (includes uptime info)

Version Endpoint

• Add /api/1/version endpoint exposing version and build information

Improved Error Messages (Addresses #125, #124, #119)

• Document ALLOWED_HOSTS fix for "Invalid HTTP_HOST header" error
• Document OAuth callback URL requirements for "redirect_uri MUST match" error
• Document database migration commands for "Table doesn't exist" error
• Add comprehensive troubleshooting section to README

Docker Compose Improvements

• Add docker-compose.prebuilt.yml for quick deployment with pre-built images
• Add healthcheck configuration
• Document all authentication options with inline comments

Test Plan

• Test local login with LOCAL_AUTH_ENABLED=true
• Test self-registration with LOCAL_AUTH_REGISTRATION_ENABLED=true
• Test password change for logged-in users
• Test /api/1/auth/status returns correct auth methods
• Test health endpoints (/health, /ready, /live)
• Test /api/1/version endpoint
• Test proxy auth with X-Auth-User-Email header

🤖 Generated with Claude Code