feat(server): tenant-registry skeleton boots against dev stack

Minimal Go service: /healthz + /v1/tenants/by-slug/:slug + /v1/tenants/:id with an in-memory store seeded with the acme tenant. Stdlib-only; pgx + JWT validation land in M4.1 follow-up.

internal/config/config_test.go

@@ -0,0 +1,52 @@
+package config
+
+import (
+	"testing"
+)
+
+func TestLoad_defaults(t *testing.T) {
+	t.Setenv("APP_ENV", "")
+	t.Setenv("ADDR", "")
+	t.Setenv("KEYCLOAK_ISSUER", "")
+	t.Setenv("DATABASE_URL", "")
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.Env != "dev" {
+		t.Errorf("Env = %q, want dev", cfg.Env)
+	}
+	if cfg.Addr != ":8080" {
+		t.Errorf("Addr = %q, want :8080", cfg.Addr)
+	}
+	if cfg.KeycloakIssuer == "" {
+		t.Error("KeycloakIssuer is empty; expected a default")
+	}
+	if cfg.DatabaseURL != "" {
+		t.Errorf("DatabaseURL = %q, want empty default", cfg.DatabaseURL)
+	}
+}
+
+func TestLoad_overrides(t *testing.T) {
+	t.Setenv("APP_ENV", "stage")
+	t.Setenv("ADDR", ":9000")
+	t.Setenv("KEYCLOAK_ISSUER", "https://auth.example/realms/r")
+	t.Setenv("DATABASE_URL", "postgres://x")
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.Env != "stage" || cfg.Addr != ":9000" || cfg.KeycloakIssuer != "https://auth.example/realms/r" || cfg.DatabaseURL != "postgres://x" {
+		t.Errorf("overrides not applied: %+v", cfg)
+	}
+}
+
+func TestLoad_invalidEnv(t *testing.T) {
+	t.Setenv("APP_ENV", "bogus")
+	_, err := Load()
+	if err == nil {
+		t.Fatal("expected error for invalid APP_ENV")
+	}
+}