[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/handlers/api.go

@@ -12,7 +12,7 @@ import (
 
 	"git.eeqj.de/sneak/neoirc/internal/db"
 	"git.eeqj.de/sneak/neoirc/pkg/irc"
-	"github.com/go-chi/chi"
+	"github.com/go-chi/chi/v5"
 )
 
 var validNickRe = regexp.MustCompile(