Migrate from go-chi/chi v1 to go-chi/chi/v5

Replace github.com/go-chi/chi v1.5.5 with github.com/go-chi/chi/v5
v5.2.1 to resolve GO-2026-4316 (open redirect in RedirectSlashes
middleware). Update all import paths across the codebase and update
documentation references in README.md and CONVENTIONS.md.

internal/server/routes.go

@@ -8,8 +8,8 @@ import (
 	"git.eeqj.de/sneak/neoirc/web"
 
 	sentryhttp "github.com/getsentry/sentry-go/http"
-	"github.com/go-chi/chi"
-	"github.com/go-chi/chi/middleware"
+	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/spf13/viper"
 )

internal/server/server.go

@@ -20,7 +20,7 @@ import (
 	"go.uber.org/fx"
 
 	"github.com/getsentry/sentry-go"
-	"github.com/go-chi/chi"
+	"github.com/go-chi/chi/v5"
 
 	_ "github.com/joho/godotenv/autoload" // loads .env file
 )