[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.sum

@@ -18,8 +18,8 @@ github.com/gdamore/tcell/v2 v2.13.8 h1:Mys/Kl5wfC/GcC5Cx4C2BIQH9dbnhnkPgS9/wF3Rl
 github.com/gdamore/tcell/v2 v2.13.8/go.mod h1:+Wfe208WDdB7INEtCsNrAN6O2m+wsTPk1RAovjaILlo=
 github.com/getsentry/sentry-go v0.42.0 h1:eeFMACuZTbUQf90RE8dE4tXeSe4CZyfvR1MBL7RLEt8=
 github.com/getsentry/sentry-go v0.42.0/go.mod h1:eRXCoh3uvmjQLY6qu63BjUZnaBu5L5WhMV1RwYO8W5s=
-github.com/go-chi/chi v1.5.5 h1:vOB/HbEMt9QqBqErz07QehcOKHaWFtuj87tTDVz2qXE=
-github.com/go-chi/chi v1.5.5/go.mod h1:C9JqLr3tIYjDOZpzn+BCuxY8z8vmca43EeMgyZt7irw=
+github.com/go-chi/chi/v5 v5.2.1 h1:KOIHODQj58PmL80G2Eak4WdvUzjSJSm0vG72crDCqb8=
+github.com/go-chi/chi/v5 v5.2.1/go.mod h1:L2yAIGWB3H+phAw1NxKwWM+7eUH/lU8pOMm5hHcoops=
 github.com/go-chi/cors v1.2.2 h1:Jmey33TE+b+rB7fT8MUy1u0I4L+NARQlK6LhzKPSyQE=
 github.com/go-chi/cors v1.2.2/go.mod h1:sSbTewc+6wYHBBCW7ytsFSn836hqM7JxpglAy2Vzc58=
 github.com/go-errors/errors v1.4.2 h1:J6MZopCL4uSllY1OfXM374weqZFFItUbrImctkmUxIA=