refactor(handlers): standardize logging using request attributes helper

- Replace manual IP logging with the `withRequestLogAttrs` helper in authentication handlers.
- Add user activity logging for API documentation and login page views.
- Clean up log calls to use variadic expansion of request attributes.

backend/libs/handlers/logging.go

@@ -0,0 +1,29 @@
+package handlers
+
+import (
+	"net/http"
+
+	"warpbox.dev/backend/libs/middleware"
+)
+
+func requestLogAttrs(r *http.Request) []any {
+	attrs := []any{
+		"ip", uploadClientIP(r),
+		"method", r.Method,
+		"path", r.URL.Path,
+	}
+	if requestID := middleware.RequestIDFromContext(r.Context()); requestID != "" {
+		attrs = append(attrs, "request_id", requestID)
+	}
+	if userAgent := r.UserAgent(); userAgent != "" {
+		attrs = append(attrs, "user_agent", userAgent)
+	}
+	return attrs
+}
+
+func withRequestLogAttrs(r *http.Request, attrs ...any) []any {
+	out := make([]any, 0, len(attrs)+8)
+	out = append(out, attrs...)
+	out = append(out, requestLogAttrs(r)...)
+	return out
+}