feat(ldap): support anonymous bind (no bind DN/password required)

bindDn and bindPassword are now optional on LdapConfig (nullable
migration) and throughout the DTOs/service/client -- an admin can
leave both blank to connect to directories that permit anonymous
read access. LdapService.bind() falls back to an RFC 4513 anonymous
bind (empty DN + empty password) whenever either field is missing,
shared across testConnection, listGroups, and syncUsersForTenant.

Frontend: removed the required attribute from Bind-DN/Bind-Passwort,
added a placeholder hint ("leer = anonymous bind"), and the
"Verbindung testen" button now only needs a Server-URL to enable
(not bindDn+bindPassword). Config responses now return bindPassword
as null (not a misleading "********") when no password is set.

Verified locally: submitted only a Server-URL with both bind fields
empty and confirmed the request reached the anonymous-bind code path
(DNS failure for the unreachable test host, not a validation error).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/api/prisma/migrations/20260708130000_ldap_anonymous_bind/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+-- Allow anonymous LDAP binds: bindDn/bindPassword become optional.
+ALTER TABLE "LdapConfig" ALTER COLUMN "bindDn" DROP NOT NULL;
+ALTER TABLE "LdapConfig" ALTER COLUMN "bindPassword" DROP NOT NULL;