Listen address formatting does not handle IPv6 literals (host:port without brackets)

[coderdan]

Context

Surfaced in review of PR #397. Both inbound-bind paths format the listen address with naive string interpolation and no IPv6 bracketing:

• ServerConfig::to_socket_address() (packages/cipherstash-proxy/src/config/server.rs) — format!("{}:{}", self.host, self.port), used by the primary bind / retry path.
• The random-port fallback in connect::bind_with_retry (packages/cipherstash-proxy/src/connect/mod.rs) — format!("{}:0", server.host).

If the configured listen host is an IPv6 literal (e.g. ::1 or fe80::1), the result is an unparseable address like ::1:6432 / ::1:0 rather than the required [::1]:6432. So an IPv6 listen host fails to bind.

This is a pre-existing limitation (the primary path has always done this); PR #397's fallback just inherits the same pattern. The same database-side helper (DatabaseConfig::to_socket_address) has the same shape and may warrant the same treatment.

Proposed fix

Build a real SocketAddr (resolve/parse host + port) or bracket IPv6 literals before combining with the port, in one shared helper used by both the primary and fallback bind paths (and consider the database helper).

Acceptance criteria

• A configured IPv6 listen host binds successfully on both the primary and random-port-fallback paths.
• IPv4 / hostname behaviour is unchanged.
• Regression coverage for an IPv6 listen address.