[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>

internal/middleware/middleware.go

@@ -11,7 +11,7 @@ import (
 	"git.eeqj.de/sneak/neoirc/internal/globals"
 	"git.eeqj.de/sneak/neoirc/internal/logger"
 	basicauth "github.com/99designs/basicauth-go"
-	chimw "github.com/go-chi/chi/middleware"
+	chimw "github.com/go-chi/chi/v5/middleware"
 	"github.com/go-chi/cors"
 	metrics "github.com/slok/go-http-metrics/metrics/prometheus"
 	ghmm "github.com/slok/go-http-metrics/middleware"