feat: MCP tools for posts, categories and tags, the draft_post prompt

- journal.Plugin.MCP: list_posts, show_post, list_categories, list_tags,
  create_post, update_post, delete_post and the draft_post prompt, guard
  backend, listed with golem15.journal.access_posts; update fields are
  optional so only sent keys reach the route
- optionalBackendPrincipal honours the MCP dispatch marker (no bearer
  short-circuit; anonymous reads unchanged)
- require summercms v0.4.4
- TestJournalMCPSmoke, TestOptionalBackendPrincipalDispatch

controllers/api/optional_principal_test.go

@@ -0,0 +1,41 @@
+package api
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/bouncer"
+)
+
+// TestOptionalBackendPrincipalDispatch pins the D-17 fix: a request the MCP
+// endpoint dispatches in-process carries no Authorization header but the
+// backend dispatch marker, and the list and show handlers must see its
+// principal; a request with neither stays anonymous.
+func TestOptionalBackendPrincipalDispatch(t *testing.T) {
+	app := backpack.New(nil)
+	admin := &bouncer.Principal{ID: 42, Backend: true}
+
+	req := httptest.NewRequest(http.MethodGet, "/_journal/api/v1/posts", nil)
+	req = req.WithContext(bouncer.WithDispatch(req.Context(), &bouncer.Dispatch{Guards: []string{"backend"}, Principal: admin}))
+	out := optionalBackendPrincipal(app, req)
+	if pr, ok := bouncer.User(out.Context()); !ok || pr != admin {
+		t.Fatalf("dispatched request principal = %+v (%v), want the marker's admin", pr, ok)
+	}
+
+	anon := httptest.NewRequest(http.MethodGet, "/_journal/api/v1/posts", nil)
+	if out := optionalBackendPrincipal(app, anon); out != anon {
+		t.Fatal("a request without a bearer or marker was changed")
+	}
+	if _, ok := bouncer.User(anon.Context()); ok {
+		t.Fatal("an anonymous request gained a principal")
+	}
+
+	// A marker for another guard is not the backend's.
+	other := httptest.NewRequest(http.MethodGet, "/_journal/api/v1/posts", nil)
+	other = other.WithContext(bouncer.WithDispatch(other.Context(), &bouncer.Dispatch{Guards: []string{"api_token"}, Principal: &bouncer.Principal{ID: 7}}))
+	if out := optionalBackendPrincipal(app, other); out != other {
+		t.Fatal("a marker of another guard yielded a backend principal")
+	}
+}