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.

prisma/migrations/20260304000000_normalize_local_usernames/migration.sql

@@ -0,0 +1,3 @@
+-- Normalize existing local usernames to lowercase
+UPDATE users SET plex_username = LOWER(plex_username) WHERE auth_provider = 'local' AND deleted_at IS NULL;
+UPDATE users SET plex_id = 'local-' || LOWER(SUBSTRING(plex_id FROM 7)) WHERE plex_id LIKE 'local-%' AND plex_id NOT LIKE 'local-%-deleted-%';