add migration docs M0004 for isolating http handlers

migrations.md

@@ -3,6 +3,75 @@
 This file contains a list of tasks that are either required or at least
 strongly recommended to align projects using this SDK.
 
+## M0004 2024-09-13 Isolate HTTP handlers
+
+### Reasoning
+
+Our previous approach to have a single `Server` struct and to put all logic there gets messy pretty fast. Worse than
+that it is hard to refactor this to separate this into multiple structs or packages. Dependency injection will help us
+to separate those things in the very beginning of a project without manual wiring of dependencies. One step towards this
+to split up all HTTP handlers into separate files.
+
+### Hints
+
+* The handlers should be moved into the `pkg/app/handlers` package.
+* The handler struct should have a `New...` constructor with all required dependencies as parameters.
+* The handler struct should implement `interface { Register(chi.Router) }`, which gets called once to set up the routes.
+  This will later be used for dependency injection integration.
+
+### Example
+
+```go
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/rebuy-de/platform-inventory/pkg/bll/webutilext"
+	"github.com/rebuy-de/platform-inventory/pkg/dal/sqlc"
+	"github.com/rebuy-de/rebuy-go-sdk/v8/pkg/webutil"
+)
+
+type KubeEventHandler struct {
+	sqlc   *sqlc.Queries
+	viewer *webutilext.JetViewer
+}
+
+func NewKubeEventHandler(
+    sqlc *sqlc.Queries,
+    viewer *webutilext.JetViewer,
+) *KubeEventHandler {
+	return &KubeEventHandler{
+		sqlc:   sqlc,
+		viewer: viewer,
+	}
+}
+
+func (h *KubeEventHandler) Register(router chi.Router) {
+	router.Get("/kube/events", webutilext.WrapView(h.list))
+	router.Get("/kube/events/table-fragment", webutilext.WrapView(h.listFragment))
+}
+
+func (h *KubeEventHandler) list(r *http.Request) webutil.Response {
+	events, err := h.sqlc.ListKubeEvents(r.Context())
+	if err != nil {
+		return webutilext.ViewError(http.StatusInternalServerError, err)
+	}
+
+	return h.viewer.HTML(http.StatusOK, "kube_event_list.html", events)
+}
+
+func (h *KubeEventHandler) listFragment(r *http.Request) webutil.Response {
+	events, err := h.sqlc.ListKubeEvents(r.Context())
+	if err != nil {
+		return webutilext.ViewError(http.StatusInternalServerError, err)
+	}
+
+	return h.viewer.HTML(http.StatusOK, "frames/kube_event_table.html", events)
+}
+```
+
 ## M0003 2024-08-16 Change viewer interfaces of webutil
 
 ### Reasoning