refactor(api): consolidate health check endpoints to /health

Removes the redundant `/healthz` and `/api/v1/health` endpoints, leaving `/health` as the sole health check endpoint.

- Update router to return 404 Not Found for the removed endpoints
- Update admin log filtering to only ignore `/health`
- Remove health URL from API documentation data
- Update tests to verify `/health` returns 200 and others return 404
- Update README documentation to reflect the change

backend/libs/handlers/app.go

@@ -129,8 +129,8 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /d/{boxID}/thumb/{fileID}", a.Thumbnail)
 	mux.HandleFunc("GET /d/{boxID}/og-image.jpg", a.BoxOGImage)
 	mux.HandleFunc("GET /health", a.Health)
-	mux.HandleFunc("GET /healthz", a.Health)
-	mux.HandleFunc("GET /api/v1/health", a.Health)
+	mux.HandleFunc("GET /healthz", notFound)
+	mux.HandleFunc("GET /api/v1/health", notFound)
 	mux.HandleFunc("GET /api/v1/sharex/warpbox-anonymous.sxcu", a.ShareXAnonymousConfig)
 	mux.HandleFunc("GET /api/v1/schemas/upload-request.json", a.UploadRequestSchema)
 	mux.HandleFunc("GET /api/v1/schemas/upload-response.json", a.UploadResponseSchema)
@@ -138,3 +138,7 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /emoji/{pack}/{file}", a.EmojiAsset)
 	mux.Handle("GET /static/", a.Static())
 }
+
+func notFound(w http.ResponseWriter, r *http.Request) {
+	http.NotFound(w, r)
+}