feat: batches with dry run, undo, and CSV import and export

- POST /_seo/api/v1/batch runs changes in one transaction: dry_run
  answers the diff and the conflicts (loops, chains, noindex on a
  published record, canonicals to redirected or noindex addresses) and
  rolls back; a refused change refuses the batch (D-12)
- undo of a change or a whole batch from History and through MCP, with
  the undo_stale, undo_loop and undo_gone refusals (D-13)
- CSV export and import of redirects, rules and records by URL: BOM,
  comma or semicolon, formula cells escaped, a dry-run preview, all or
  nothing with via import; the screen's apply token is HMAC-SHA256 over
  the file and a fingerprint of the data (transfer.stale)
- Import and export singleton with the seo_transfer widget action,
  History record actions undo and undo_batch, navigation entry
- MCP tools undo_change, undo_batch, apply_batch, export_csv, import_csv
- TestBatch, TestUndo, TestCSVTransfer, TestLoopRefused mcp and import

models/transfer.go

@@ -0,0 +1,24 @@
+package models
+
+import "time"
+
+// Transfer is the import and export screen's view of the settings row
+// (id 1 on golem15_seo_settings, UI-SPEC C13): the CSV delimiter its
+// footer Save stores. It is not listed in All, because the row's model is
+// Settings.
+type Transfer struct {
+	ID           uint       `gorm:"column:id;primaryKey"`
+	CSVDelimiter string     `gorm:"column:csv_delimiter"`
+	UpdatedAt    *time.Time `gorm:"column:updated_at"`
+}
+
+func (Transfer) TableName() string { return "golem15_seo_settings" }
+
+// MorphName names the screen's record.
+func (Transfer) MorphName() string { return `Golem15\SEO\Models\Transfer` }
+
+// Fillable is the delimiter.
+func (Transfer) Fillable() []string { return []string{"csv_delimiter"} }
+
+// Rules is empty: the controller checks the delimiter.
+func (Transfer) Rules() map[string]string { return map[string]string{} }