feat: graph validation and custom JSON-LD

- seo_schema (seoschema, E8) with the C17 server rules: invalid JSON with
  line and column, non-objects, 16 KB, depth 8, a literal <script and a
  type outside the kind's list refused; the value re-serialised
- a custom node with a generated @id merges into it (schema.merged save
  notice), other nodes are appended to the graph
- seo_primary_parent (seoparent) for article types; both values stored
  in the entry data on every locale row
- the seo_graph widget action (E9 data), P1 primary_gone and the P2
  schema type and breadcrumb trail rows
- GET /_seo/api/v1/graph and MCP seo_get_graph; seo_set_url_seo and
  apply_batch accept primary_parent and schema

The graph tool lands with the validation: the operations parity test
binds the seo_graph widget action to its tool.

operations.go

@@ -51,6 +51,7 @@ var AdminOperations = []AdminOperation{
 	{ContentController, OpShow, "get_url_seo"},
 	{ContentController, OpExtensionSave, "set_url_seo"},
 	{ContentController, OpWidgetAction + ":seo_snippet", "get_url_seo"},
+	{ContentController, OpWidgetAction + ":seo_graph", "get_graph"},
 	// The social image is set by URL through MCP (uploads are no tool).
 	{ContentController, OpUpload, "set_url_seo"},