feat: revision-check the upsert, MCP and admin writes

- POST /materials locks the matched row and answers 409 to a stale
  revision when it updates; a create ignores revision
- bm_manage_save_material and bm_manage_update_material take revision
- /bmadmin saves bump the revision on content changes (not checked)
- document revisions and the 409 in the README

controllers/materials_hooks_test.go

@@ -2,10 +2,13 @@ package controllers
 
 import (
 	"context"
+	"errors"
 	"slices"
 	"strconv"
 	"testing"
+	"time"
 
+	"git.golem15.com/golem15/summercms/modules/cabana"
 	"git.golem15.com/jakub/sm-bm-plugin/models"
 	"gorm.io/gorm"
 )
@@ -86,3 +89,42 @@ func TestMaterialsAdminHooks(t *testing.T) {
 		t.Errorf("calls for a wrong model = %v", rec.calls)
 	}
 }
+
+// TestMaterialsAdminRevision checks the revision hooks of the materials
+// admin outside a database write: without a transaction they do nothing and
+// store no snapshot, a model that is not a material is an error, and the
+// embedded date check still runs first.
+func TestMaterialsAdminRevision(t *testing.T) {
+	t.Cleanup(func() { UseMaterialIndex(nil) })
+	UseMaterialIndex(nil)
+	var c materialsAdmin
+	ctx := context.Background()
+	m := &models.Material{ID: 41, Title: "Notatka", Type: "notatka"}
+
+	if err := c.FormBeforeUpdate(ctx, m); err != nil {
+		t.Errorf("before update without a transaction: %v", err)
+	}
+	if _, found := pendingRevisions.Load(m.ID); found {
+		t.Error("before update without a transaction stored a snapshot")
+	}
+	if err := c.FormAfterUpdate(ctx, m); err != nil {
+		t.Errorf("after update without a transaction: %v", err)
+	}
+
+	if err := c.FormBeforeUpdate(ctx, &models.Course{ID: 41}); err == nil {
+		t.Error("before update with a course: no error")
+	}
+	var nilMaterial *models.Material
+	if err := c.FormBeforeUpdate(ctx, nilMaterial); err == nil {
+		t.Error("before update with a nil material: no error")
+	}
+
+	at := func(h, m int) time.Time { return time.Date(2026, 10, 3, h, m, 0, 0, models.Location) }
+	bad := &models.ClassSession{StartsAt: at(8, 0), EndsAt: at(7, 0)}
+	var ve *cabana.ValidationError
+	if err := c.FormBeforeUpdate(ctx, bad); !errors.As(err, &ve) {
+		t.Errorf("before update with bad dates: err = %v, want a cabana.ValidationError", err)
+	} else if _, ok := ve.Details["ends_at"]; !ok {
+		t.Errorf("before update with bad dates: details %v name no ends_at", ve.Details)
+	}
+}