feat(subsonic): mount /rest router with /ping and /getLicense

Exposes each handler at /rest/name and /rest/name.view, GET+POST, for
client convention compatibility.

internal/subsonic/subsonic.go

@@ -0,0 +1,34 @@
+// Package subsonic implements a Subsonic-compatible REST API surface under
+// /rest. It exists for third-party Subsonic clients (Symfonium, Substreamer,
+// DSub, Tempo, play:Sub, …). Minstrel's own clients use the native /api/*
+// surface, which has modern auth; /rest/* retains Subsonic's legacy auth for
+// compatibility only. See docs/auth.md for the security posture.
+package subsonic
+
+import (
+	"log/slog"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// Mount attaches Subsonic handlers at /rest on r. Endpoints are exposed at
+// both /rest/foo and /rest/foo.view because client conventions vary. Both
+// GET and POST are accepted; Subsonic's auth params live in the query string
+// either way.
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, cfg Config) {
+	r.Route("/rest", func(sub chi.Router) {
+		sub.Use(Middleware(pool, cfg))
+		register(sub, "/ping", handlePing)
+		register(sub, "/getLicense", handleGetLicense)
+		_ = logger
+	})
+}
+
+func register(r chi.Router, path string, h func(http.ResponseWriter, *http.Request)) {
+	r.Get(path, h)
+	r.Post(path, h)
+	r.Get(path+".view", h)
+	r.Post(path+".view", h)
+}