Add LDAP authentication

- LDAP bind + search auth with auto-create local user
- Falls back to local auth if LDAP not configured or fails
- Configurable via MH_LDAP_* environment variables
- Supports ldap:// and ldaps:// with optional TLS skip
- go-ldap/ldap/v3 dependency added

go.mod

@@ -3,6 +3,7 @@ module markdownhub
 go 1.23
 
 require (
+	github.com/go-ldap/ldap/v3 v3.4.8
 	github.com/golang-jwt/jwt/v5 v5.2.1
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/websocket v1.5.3
@@ -11,7 +12,9 @@ require (
 )
 
 require (
+	github.com/Azure/go-ntlmssp v0.0.0-20221128193559-754e69321358 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/go-asn1-ber/asn1-ber v1.5.5 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v0.1.9 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect