fix: supervise the cowboy listener instead of starting it as a side effect

[Taure]

Problem

nova_sup:init/1 starts cowboy by calling cowboy:start_clear/3 (or start_tls/3) as a side effect and discards the result:

setup_cowboy(Configuration),   %% return value ignored
{ok, {SupFlags, Children}}.

setup_cowboy/1 logs {error, Reason} and returns. So when the listener can't bind (e.g. another node already holds the port, eaddrinuse), nova logs Cowboy could not start but application:start(nova) still returns {ok, ...}. The node comes up "healthy" with no working listener, and requests silently hit whatever already owns the port.

This is a nasty footgun in development (a stale make start/shell node holding 8080) and in production (port conflict on deploy): ensure_all_started(my_app) succeeds, health checks may pass, but no route resolves. The symptom looks exactly like a routing bug (manual routing_tree:lookup returns {ok, ...} in the new node, but the request 404s because it landed on the other node).

Fix

Build the listener with ranch:child_spec/5 and add it to nova's supervision tree instead of starting it as a side effect:

• A failed bind now fails init/1 and surfaces through application:start/1 (loud, not swallowed).
• The listener is restart-managed like any other child.

The transport/protocol option transforms (connection_type, dynamic buffer) mirror cowboy:start_clear/start_tls for cowboy 2.15 (the pinned version) so socket behaviour is identical to before.

Before / after

%% port already held by another node
%% before:
application:ensure_all_started(my_app).
{ok, [...]}                         %% listener silently dead

%% after:
application:ensure_all_started(my_app).
{error, {nova, {{shutdown,
   {failed_to_start_child, {ranch_embedded_sup, nova_listener},
    ...{listen_error, nova_listener, eaddrinuse}}}, {nova_app, start, [normal, []]}}}}

Tests

test/nova_sup_tests.erl:

• child_spec_is_supervised_test / tls_child_spec_uses_ssl_transport_test — the listener is a supervisor:child_spec() with type => supervisor and connection_type => supervisor.
• listener_lifecycle_test_ — a free port yields a live listener; a busy port fails the start instead of being swallowed.

Full suite green: eunit 351/0, xref clean, dialyzer clean. Also dogfooded through a downstream Nova app: the listener-bind failure now correctly aborts that app's startup.