feat(auth): replace .env/godotenv secret handling with config-based secrets

Secrets (JWT keys, LDAP sync password, OIDC client id/secret, cross-login
keys) are now configured directly in config.json under the auth section
where they are used. Each secret can still be supplied via its existing
environment variable, which takes precedence over the config value.

The godotenv dependency, the .env file, configs/env-template.txt and the
loadEnvironment() bootstrap step are removed. -init now writes the demo
JWT keys into config.json instead of a .env file.

Closes #283

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: 3a7cb814c53f

internal/auth/auth.go

@@ -17,6 +17,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"os"
 	"sync"
 	"time"
 
@@ -115,6 +116,18 @@ type AuthConfig struct {
 // Keys holds the global authentication configuration
 var Keys AuthConfig
 
+// secretFromEnv resolves a secret from the environment or config. The
+// environment variable takes precedence when set and non-empty; otherwise the
+// value configured in config.json is used. This lets deployments inject secrets
+// via the environment (or a secret manager) while keeping config.json
+// self-contained for simple setups.
+func secretFromEnv(envVar, configValue string) string {
+	if v := os.Getenv(envVar); v != "" {
+		return v
+	}
+	return configValue
+}
+
 // Authentication manages all authentication methods and session handling
 type Authentication struct {
 	sessionManager *scs.SessionManager