fix: Add /health endpoint for Fly.io health checks

[StuartF303]

Added health check endpoint to resolve 404 errors during deployment:

• Added GET /health endpoint returning status and timestamp
• Returns 200 OK with JSON response: {"status":"healthy","timestamp":"..."}

Also improved CORS configuration:

• Development: Strict CORS with specific origins (localhost)
• Production: Permissive CORS (traffic comes through nginx on same domain)

Fixes health check failures seen in Fly.io logs:
"Request reached the end of the middleware pipeline without being handled"

The health endpoint is proxied through nginx at /health on port 8080.