Normalize local usernames to lowercase

Normalize local account usernames by trimming and lowercasing across the stack. Added a Prisma migration to lowercase existing plex_username and rewrite local plex_id values for non-deleted accounts. Updated LocalAuthProvider, admin login route, and setup completion to use normalized usernames when looking up, creating, and storing users (including plexId `local-{username}`). Added/updated tests to assert case-insensitive lookups, storage of lowercased usernames/plexIds, and duplicate username rejection.

src/app/api/auth/admin/login/route.ts

@@ -38,9 +38,11 @@ export async function POST(request: NextRequest) {
       );
     }
 
+    const normalizedUsername = username.trim().toLowerCase();
+
     // Find user by local admin identifier
     const user = await prisma.user.findUnique({
-      where: { plexId: `local-${username}` },
+      where: { plexId: `local-${normalizedUsername}` },
     });
 
     if (!user) {