Migrate config to mcdsl: Load[T], env overrides, embedded types

Replace local TOML loading with mcdsl/config.Load[Config] which adds
METACRYPT_ environment variable overrides. ServerConfig and MCIASConfig
now embed their mcdsl counterparts, extending with ExternalURL and
ServiceToken respectively. DatabaseConfig and LogConfig replaced with
mcdsl types directly.

TOML structure is preserved — no config file changes needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/config_test.go

@@ -41,6 +41,38 @@ server_url = "https://mcias.example.com"
 	}
 }
 
+func TestLoadEnvOverride(t *testing.T) {
+	content := `
+[server]
+listen_addr = ":8443"
+tls_cert = "cert.pem"
+tls_key = "key.pem"
+
+[database]
+path = "test.db"
+
+[mcias]
+server_url = "https://mcias.example.com"
+`
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.toml")
+	_ = os.WriteFile(path, []byte(content), 0600)
+
+	t.Setenv("METACRYPT_SERVER_LISTEN_ADDR", ":9999")
+	t.Setenv("METACRYPT_MCIAS_SERVER_URL", "https://override.example.com")
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Errorf("ListenAddr env override: got %q, want %q", cfg.Server.ListenAddr, ":9999")
+	}
+	if cfg.MCIAS.ServerURL != "https://override.example.com" {
+		t.Errorf("ServerURL env override: got %q", cfg.MCIAS.ServerURL)
+	}
+}
+
 func TestLoadMissingRequired(t *testing.T) {
 	content := `
 [server]