Address second review findings F1-F3 for 404, public URL and listen host

- F1: return the JSON API error shape for unknown paths (404 not_found)
- F2: require a non-empty hostname in MEALQUEST_PUBLIC_URL
- F3: validate the MEALQUEST_HTTP_ADDR host as an IP address or hostname

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/config/config_test.go

@@ -102,6 +102,43 @@ func TestLoad(t *testing.T) {
 			env:     with(map[string]string{EnvHTTPAddr: ":65536"}),
 			badVars: []string{EnvHTTPAddr},
 		},
+		{
+			name: "listen address with IP host",
+			env:  with(map[string]string{EnvHTTPAddr: "127.0.0.1:8080"}),
+		},
+		{
+			name: "listen address with IPv6 host",
+			env:  with(map[string]string{EnvHTTPAddr: "[::1]:8080"}),
+		},
+		{
+			name: "listen address with hostname",
+			env:  with(map[string]string{EnvHTTPAddr: "localhost:8080"}),
+		},
+		{
+			name:    "listen host with space",
+			env:     with(map[string]string{EnvHTTPAddr: "bad host:8080"}),
+			badVars: []string{EnvHTTPAddr},
+		},
+		{
+			name:    "listen host with leading hyphen label",
+			env:     with(map[string]string{EnvHTTPAddr: "-bad.example.com:8080"}),
+			badVars: []string{EnvHTTPAddr},
+		},
+		{
+			name:    "listen host with empty label",
+			env:     with(map[string]string{EnvHTTPAddr: "bad..example.com:8080"}),
+			badVars: []string{EnvHTTPAddr},
+		},
+		{
+			name:    "listen host with label over 63 chars",
+			env:     with(map[string]string{EnvHTTPAddr: strings.Repeat("a", 64) + ".example.com:8080"}),
+			badVars: []string{EnvHTTPAddr},
+		},
+		{
+			name:    "public URL without hostname",
+			env:     with(map[string]string{EnvPublicURL: "http://:8080"}),
+			badVars: []string{EnvPublicURL},
+		},
 		{
 			name:    "relative public URL",
 			env:     with(map[string]string{EnvPublicURL: "meals.example.com"}),