deps: migrate from go-chi/chi v1 to go-chi/chi/v5

Migrate all import paths from github.com/go-chi/chi to
github.com/go-chi/chi/v5 and github.com/go-chi/chi/middleware
to github.com/go-chi/chi/v5/middleware.

This resolves GO-2026-4316 (open redirect vulnerability in
RedirectSlashes middleware) by moving to the maintained v5
module path.

Files changed:
- go.mod: chi v1.5.5 → chi/v5 v5.2.1
- internal/server/server.go: updated import
- internal/server/routes.go: updated imports
- internal/middleware/middleware.go: updated import
- internal/handlers/api.go: updated import
- README.md: updated Required Libraries table

README.md

@@ -2336,7 +2336,7 @@ neoirc/
 | Purpose    | Library |
 |------------|---------|
 | DI         | `go.uber.org/fx` |
-| Router     | `github.com/go-chi/chi` |
+| Router     | `github.com/go-chi/chi/v5` |
 | Logging    | `log/slog` (stdlib) |
 | Config     | `github.com/spf13/viper` |
 | Env        | `github.com/joho/godotenv/autoload` |