deps: migrate from chi v1 to chi/v5

Migrate all chi imports from github.com/go-chi/chi (v1.5.5) to
github.com/go-chi/chi/v5 (v5.2.1) to resolve GO-2026-4316 open
redirect vulnerability in the RedirectSlashes middleware.

Changes:
- go.mod: replaced chi v1.5.5 with chi/v5 v5.2.1
- Updated import paths in server, routes, middleware, and handlers
- No API changes required (chi/v5 is API-compatible for our usage)

closes #42

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8
 	github.com/gdamore/tcell/v2 v2.13.8
 	github.com/getsentry/sentry-go v0.42.0
-	github.com/go-chi/chi v1.5.5
+	github.com/go-chi/chi/v5 v5.2.1
 	github.com/go-chi/cors v1.2.2
 	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1