fix(auth): make usernames case-insensitive

Username lookups (login, admin seed, LDAP sync) compared case-sensitively
against a stored value with whatever casing it was created with, so
"Admin" and "admin" were treated as different accounts.

Normalizes at every write and read path: UserService.create/update
lowercase the username before persisting, findByUsername lowercases
the lookup input, AuthService.validateUser lowercases before the login
query, AdminSeedService lowercases the configured admin username, and
the LDAP sync loop lowercases the mapped sAMAccountName before using it
for lookup/create/update -- so AD casing differences don't create
duplicate accounts either.

Added a data migration to lowercase any existing mixed-case usernames.
It relies on the User.username unique constraint to fail loudly if two
existing accounts would collide after normalizing, rather than silently
merging them.

Verified locally: logged in with "ADMIN" (uppercase) against the
existing lowercase "admin" account after rebuilding the API image.

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

apps/api/src/ldap/ldap.service.ts

@@ -223,8 +223,9 @@ export class LdapService {
             }
           }
 
-          // Require at minimum a username
-          const username = mappedData['username'];
+          // Require at minimum a username. Normalize to lowercase so
+          // logins stay case-insensitive regardless of AD casing.
+          const username = mappedData['username']?.toLowerCase();
           if (!username) {
             result.errors.push(
               `Entry ${dn}: no username mapped (check sAMAccountName mapping)`,
@@ -254,9 +255,7 @@ export class LdapService {
                   displayName: mappedData['displayName'],
                 }),
                 ...(mappedData['email'] && { email: mappedData['email'] }),
-                ...(mappedData['username'] && {
-                  username: mappedData['username'],
-                }),
+                ...(mappedData['username'] && { username }),
                 ldapDn: dn,
                 isActive: true,
               },