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/graph.go

@@ -0,0 +1,48 @@
+package api
+
+import (
+	"context"
+	"errors"
+	"net/http"
+
+	"git.golem15.com/golem15/sm-seo-plugin/classes"
+)
+
+// GetGraphInput is the query of GET /_seo/api/v1/graph.
+type GetGraphInput struct {
+	URL    string  `json:"url" query:"url" jsonschema:"the page: a path such as /blog/post or an absolute URL of the site"`
+	Schema *string `json:"schema,omitempty" query:"schema" jsonschema:"an unsaved structured data value (the seo_set_url_seo schema string) to preview instead of the stored one; omitted: the stored value"`
+}
+
+// Graph serves GET /_seo/api/v1/graph?url=&schema= (D-46, D-10): the
+// JSON-LD @graph the resolve answer of the URL carries, built with the
+// given unsaved schema value when sent, and every node with its missing
+// required and recommended properties (rules false for a type Google has
+// no rules for). It answers {locale, url, graph, nodes}; url and graph are
+// null for an address that is not an ok page. 422 for a URL that is not a
+// path or http(s) URL, or for a schema value the SEO tab would refuse.
+func Graph(service func() *classes.Service) http.HandlerFunc {
+	return manage(service, func(ctx context.Context, svc *classes.Service, r *http.Request) (int, any, error) {
+		q := r.URL.Query()
+		var override *string
+		if q.Has("schema") {
+			v := q.Get("schema")
+			override = &v
+		}
+		answer, res, err := svc.GraphOfURL(ctx, q.Get("url"), override)
+		if errors.Is(err, classes.ErrBadURL) {
+			return 0, nil, Fail(http.StatusUnprocessableEntity, err.Error())
+		}
+		if err != nil {
+			return 0, nil, err
+		}
+		if override != nil && res.TypeKey != "" {
+			if typ, ok := svc.Registry().ContentType(res.TypeKey); ok && typ.Content != nil {
+				if err := svc.CheckSchema(ctx, typ.Content.Kind, res.TypeKey, res.RecordID, *override); err != nil {
+					return 0, nil, err
+				}
+			}
+		}
+		return http.StatusOK, answer, nil
+	})
+}