Add listen_address config for IP binding

[8144225309]

Problem

The server currently binds to all interfaces (in6addr_any / 0.0.0.0). On a VPS with multiple IP addresses, there's no way to bind sendrawtx to a specific IP while other services use a different IP.

Current workaround

Use iptables PREROUTING REDIRECT to forward traffic from a specific IP:port to the server's port. This works perfectly (kernel-level, zero overhead, no URL size issues) but requires external firewall rules.

Proposed solution

Add an optional listen_address config option:

[server]
port = 8080
listen_address = 203.0.113.20

The server already uses dual-stack IPv6 sockets, so this would support:

• IPv4 addresses (203.0.113.20)
• IPv6 addresses (2001:db8::1)
• All interfaces (0.0.0.0 or ::, current default when omitted)

Implementation notes

• Change is ~10 lines in worker.c (replace in6addr_any with parsed address) and config.c/config.h (add field + parser)
• Use inet_pton() to parse the address, try IPv4 first then IPv6
• Default to current behavior (all interfaces) when omitted
• The iptables workaround is fine for now — this is a nice-to-have for self-documenting configs