Validate genre names against HasMaxLength(100); return 400 on >100-char genre (currently 500 on PG)

[mforce]

Follow-up from @codex P2 on PR #173 (thread 3847543484) — genre normalization passes names >100 chars unchanged, but
CollectifyDbContext configures Genre.Name with HasMaxLength(100). PostgreSQL rejects the write at SaveChangesAsync → 500;
SQLite accepts data violating the model constraint. Fix: validate normalized genre names in GenreResolver and return 400
before creating Genre rows (do the same for create/update/bulk paths). Not a blocker for the merged PR #173; fast-follow.