feat(server): add LISTEN_ADDR env var for allowing server to listen on ipv6 (#14211)

The old code hardcoded 0.0.0.0 which means the server only listened for
ipv4 connections, making it not work on ipv6-only networks.

This change adds a LISTEN_ADDR env var which allows the server to bind
to ipv6 as well.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Server listen address is now configurable via the LISTEN_ADDR
environment variable (default: 0.0.0.0), enabling IPv4/IPv6 or
interface-specific binding.
* Configuration schemas and admin UI now expose the listen address
option so deployments can view and override it.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.docker/selfhost/schema.json

@@ -595,6 +595,11 @@
           "description": "Multiple hosts the server will accept requests from.\n@default []",
           "default": []
         },
+        "listenAddr": {
+          "type": "string",
+          "description": "The address to listen on (e.g., 0.0.0.0 for IPv4, :: for IPv6).\n@default \"0.0.0.0\"\n@environment `LISTEN_ADDR`",
+          "default": "0.0.0.0"
+        },
         "port": {
           "type": "number",
           "description": "Which port the server will listen on.\n@default 3010\n@environment `AFFINE_SERVER_PORT`",