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>

go.mod

@@ -6,7 +6,6 @@ require (
 	git.wntrmute.dev/kyle/goutils v1.21.0
 	git.wntrmute.dev/kyle/mcdsl v1.0.0
 	github.com/go-chi/chi/v5 v5.2.5
-	github.com/pelletier/go-toml/v2 v2.3.0
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/viper v1.21.0
 	golang.org/x/crypto v0.49.0
@@ -23,6 +22,7 @@ require (
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/pelletier/go-toml/v2 v2.3.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect