[deps] Migrate from chi v1 to chi/v5 (#73)

## Summary

Migrates all `go-chi/chi` imports from v1 (v1.5.5) to v5 (v5.2.1) to resolve **GO-2026-4316**, an open redirect vulnerability in the `RedirectSlashes` middleware.

## Changes

- `go.mod`: replaced `github.com/go-chi/chi v1.5.5` with `github.com/go-chi/chi/v5 v5.2.1`
- Updated import paths in 4 files:
  - `internal/server/server.go`
  - `internal/server/routes.go`
  - `internal/middleware/middleware.go`
  - `internal/handlers/api.go`
- `go.sum` updated via `go mod tidy`
- No API changes required — chi/v5 is API-compatible for all patterns used (router, middleware, URLParam)

## Verification

- `go mod tidy` ✅
- `make fmt` ✅
- `docker build .` (runs `make check`: lint, fmt-check, test) ✅
- All tests pass with 58.1% handler coverage, 100% IRC numerics coverage

closes #42

Reviewed-on: #73
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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