feat(cli): configurable bind host (--host flag + config host field)

[addadi]

What

Bind address was hardcoded to 127.0.0.1. This adds:

• Config.host JSON field (default 127.0.0.1; empty value falls back to default)
• --host <addr> CLI flag
• Precedence: CLI --host > config file host > default — mirrors the existing --port behavior
• Startup log / listen error messages now include the resolved host

Why

Multi-host and container deployments (e.g. reaching the server from another host or a container bridge) previously required a recompile to change the bind address. --port was already configurable; --host was the missing half.

Validation

• zig build clean
• zig build test green (new tests: default host, config-file host parse, host-unset keeps default)
• Manual run: nulltickets --host 127.0.0.2 --port 7799 binds and serves /health on the given address

No behavior change for existing deployments: without --host and without a config host key, the server still binds 127.0.0.1.