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.

controllers/api/seo.go

@@ -20,6 +20,8 @@ type SEOValues struct {
 	OGTitle       *string `json:"og_title,omitempty" jsonschema:"the social title"`
 	OGDescription *string `json:"og_description,omitempty" jsonschema:"the social description"`
 	OGImage       *string `json:"og_image,omitempty" jsonschema:"the social image: a path or an absolute URL"`
+	PrimaryParent *string `json:"primary_parent,omitempty" jsonschema:"records only: the primary category of the breadcrumb trail and the article section, one of the record's parents as <type>:<id> (seo_get_url_seo and the graph tool name them); empty for the first parent. Shared by every locale"`
+	Schema        *string `json:"schema,omitempty" jsonschema:"records only: the structured data as a JSON string {\"type\": schema type or empty for the default, \"fields\": {serviceType, areaServed, price, currency} for Service, \"custom\": custom JSON-LD text (an object or a list, at most 16 KB, no script tag; a node with a generated @id is merged into it)}; empty resets it. Shared by every locale"`
 }
 
 // Map returns the values that are set, keyed by column.
@@ -29,6 +31,7 @@ func (v SEOValues) Map() map[string]string {
 		"title": v.Title, "description": v.Description, "keyphrase": v.Keyphrase,
 		"canonical": v.Canonical, "robots": v.Robots, "og_title": v.OGTitle,
 		"og_description": v.OGDescription, "og_image": v.OGImage,
+		"primary_parent": v.PrimaryParent, "schema": v.Schema,
 	} {
 		if p != nil {
 			out[column] = *p