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/server/server_test.go

@@ -16,6 +16,9 @@ import (
 	"git.wntrmute.dev/kyle/metacrypt/internal/barrier"
 	"git.wntrmute.dev/kyle/metacrypt/internal/config"
 	"git.wntrmute.dev/kyle/metacrypt/internal/crypto"
+
+	mcdslauth "git.wntrmute.dev/kyle/mcdsl/auth"
+	mcdslconfig "git.wntrmute.dev/kyle/mcdsl/config"
 	"git.wntrmute.dev/kyle/metacrypt/internal/db"
 	"git.wntrmute.dev/kyle/metacrypt/internal/engine"
 	"git.wntrmute.dev/kyle/metacrypt/internal/policy"
@@ -46,12 +49,14 @@ func setupTestServer(t *testing.T) (*Server, *seal.Manager, chi.Router) {
 
 	cfg := &config.Config{
 		Server: config.ServerConfig{
-			ListenAddr: ":0",
-			TLSCert:    "cert.pem",
-			TLSKey:     "key.pem",
+			ServerConfig: mcdslconfig.ServerConfig{
+				ListenAddr: ":0",
+				TLSCert:    "cert.pem",
+				TLSKey:     "key.pem",
+			},
 		},
-		Database: config.DatabaseConfig{Path: filepath.Join(dir, "test.db")},
-		MCIAS:    config.MCIASConfig{ServerURL: "https://mcias.test"},
+		Database: mcdslconfig.DatabaseConfig{Path: filepath.Join(dir, "test.db")},
+		MCIAS:    config.MCIASConfig{Config: mcdslauth.Config{ServerURL: "https://mcias.test"}},
 		Seal: config.SealConfig{
 			Argon2Time:    1,
 			Argon2Memory:  64 * 1024,