feat: initial shared auth module

Signed user-header middleware (UserMiddleware/FromContext/User, ADR-0005) plus
the deployed-secrets startup guard (MissingDeployedSecrets, ADR-0005/0006).
Replaces the byte-identical auth package + secrets_guard.go copied into every
backend service.

secrets_test.go

@@ -0,0 +1,22 @@
+package auth
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMissingDeployedSecrets(t *testing.T) {
+	required := func(signing, internal string) map[string]string {
+		return map[string]string{"USER_SIGNING_KEY": signing, "INTERNAL_API_KEY": internal}
+	}
+	for _, env := range []string{"development", "", "acctest", "test"} {
+		assert.Nil(t, MissingDeployedSecrets(env, required("", "")), "env %q must not enforce", env)
+	}
+	assert.Nil(t, MissingDeployedSecrets("staging", required("k", "k")))
+	assert.Nil(t, MissingDeployedSecrets("production", required("k", "k")))
+	assert.Equal(t, []string{"INTERNAL_API_KEY", "USER_SIGNING_KEY"},
+		MissingDeployedSecrets("staging", required("", "")))
+	assert.Equal(t, []string{"USER_SIGNING_KEY"},
+		MissingDeployedSecrets("production", required("", "k")))
+}