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/schema.prisma

@@ -64,8 +64,8 @@ model LdapConfig {
   tenant          Tenant             @relation(fields: [tenantId], references: [id])
   serverUrl       String
   baseDn          String
-  bindDn          String
-  bindPassword    String
+  bindDn          String?
+  bindPassword    String?
   searchFilter    String             @default("(objectClass=person)")
   syncIntervalMin Int                @default(60)
   isActive        Boolean            @default(true)