server: limit wildcard CORS to the public routes (closes #100)

The CORS wildcard was global, so it also covered the Basic-Auth
protected /metrics, which REPO_POLICIES.md forbids, and it allowed
POST, PUT and DELETE, which no route serves, plus the Authorization
and X-CSRF-Token headers. CORS now sits on a router holding only the
public routes and allows GET and OPTIONS with the Accept and
Content-Type headers. /metrics gets no CORS at all.

Both are mounted routers rather than a Group: chi answers OPTIONS on
a Group's route with 405 before its middleware runs, and any method
/metrics does not register would otherwise fall through to the public
router. So every method on /metrics now meets Basic Auth first, and
/metrics/ is served like /metrics.

Model: opus-5-5

internal/server/routes_test.go

@@ -75,6 +75,17 @@ func serve(
 	return rec
 }
 
+// publicPaths returns one path on each public route.
+func publicPaths() []string {
+	return []string{
+		"/",
+		"/s/css/tailwind.min.css",
+		"/api/v1/status",
+		"/health",
+		"/.well-known/healthcheck",
+	}
+}
+
 // TestPublicRoutesAllowAnyOrigin checks that every public route answers
 // a cross-origin GET with the CORS wildcard.
 func TestPublicRoutesAllowAnyOrigin(t *testing.T) {
@@ -85,15 +96,7 @@ func TestPublicRoutesAllowAnyOrigin(t *testing.T) {
 
 	srv := routedServer(t)
 
-	paths := []string{
-		"/",
-		"/s/css/tailwind.min.css",
-		"/api/v1/status",
-		"/health",
-		"/.well-known/healthcheck",
-	}
-
-	for _, path := range paths {
+	for _, path := range publicPaths() {
 		rec := serve(srv, crossOriginRequest(t, http.MethodGet, path))
 
 		if rec.Code != http.StatusOK {
@@ -165,10 +168,13 @@ func TestMetricsHasNoCORS(t *testing.T) {
 	}
 }
 
-// TestPreflightAllowsOnlyWhatPublicRoutesServe checks what a public
+// TestPreflightAllowsOnlyWhatPublicRoutesServe checks what each public
 // route agrees to in a CORS preflight: GET, but not POST, PUT or
 // DELETE, which no route serves, and not the Authorization or
-// X-CSRF-Token headers, which no public route reads.
+// X-CSRF-Token headers, which no public route reads. It checks every
+// public route because one added with Get, such as /health, answers a
+// preflight only while CORS is middleware of a whole router; in a
+// Group, chi would answer it with 405 and no CORS headers.
 func TestPreflightAllowsOnlyWhatPublicRoutesServe(t *testing.T) {
 	viper.Reset()
 	t.Setenv("DNSWATCHER_TARGETS", "example.com")
@@ -191,23 +197,25 @@ func TestPreflightAllowsOnlyWhatPublicRoutesServe(t *testing.T) {
 		{http.MethodGet, "X-CSRF-Token", false},
 	}
 
-	for _, tt := range tests {
-		rec := serve(srv, preflightRequest(
-			t, "/api/v1/status", tt.method, tt.headers,
-		))
+	for _, path := range publicPaths() {
+		for _, tt := range tests {
+			rec := serve(srv, preflightRequest(
+				t, path, tt.method, tt.headers,
+			))
 
-		want := ""
-		if tt.allowed {
-			want = tt.method
-		}
+			want := ""
+			if tt.allowed {
+				want = tt.method
+			}
 
-		got := rec.Header().Get("Access-Control-Allow-Methods")
-		if got != want {
-			t.Errorf(
-				"preflight for %s with headers %q: "+
-					"Access-Control-Allow-Methods = %q, want %q",
-				tt.method, tt.headers, got, want,
-			)
+			got := rec.Header().Get("Access-Control-Allow-Methods")
+			if got != want {
+				t.Errorf(
+					"preflight to %s for %s with headers %q: "+
+						"Access-Control-Allow-Methods = %q, want %q",
+					path, tt.method, tt.headers, got, want,
+				)
+			}
 		}
 	}
 }