feat(16.2-07): purge the host site cache on every admin post and category write

- posts: FormAfterCreate/Update purge after the content_html translations,
  new FormAfterDelete; categories: FormAfterCreate and FormAfterDelete
- NewSiteCachePurger refuses half configuration, non-http(s) URLs, user
  info, fragments, http to a non-loopback host and tokens under 32 chars;
  Boot logs the endpoint host and path once
- TestPostsAdminPurgesSiteCache smoke test
- README: Configuration section (keys, contract, timing, scope), API rows

site_cache_admin_test.go

@@ -165,3 +165,38 @@ func TestCategoriesAdminPurgesSiteCache(t *testing.T) {
 		t.Fatalf("the purge saw content %q, want the committed %q (purge before commit)", req.Seen, edited)
 	}
 }
+
+// TestPostsAdminPurgesSiteCache proves an admin update of a post sends one
+// authorized purge request with kind post after the save commits.
+func TestPostsAdminPurgesSiteCache(t *testing.T) {
+	recv := newPurgeReceiver(t)
+	env := newPostsEnv(t, recv.env()...)
+	token := env.admin(t, controllers.PermissionAccessPosts)
+
+	rec := env.expect(t, http.StatusCreated, http.MethodPost, adminAPI(adminPostsPath), token, map[string]any{
+		"title":   map[string]any{"en": "Purge post", "pl": "Wpis"},
+		"slug":    map[string]any{"en": "purge-post", "pl": "wpis"},
+		"content": map[string]any{"en": "<p>Before</p>"},
+		"excerpt": map[string]any{"en": "Excerpt", "pl": "Wyjątek"},
+	})
+	id := adminCreatedID(t, rec)
+	recv.take()
+
+	recv.setProbe(func() string {
+		var post models.Post
+		if err := env.gdb.First(&post, id).Error; err != nil {
+			return "error: " + err.Error()
+		}
+		return post.ContentHTML
+	})
+	served := servedForm(t, env, token, id)
+	const edited = "<p>After the edit</p>"
+	body := servedBody(served, "title", "slug", "excerpt")
+	body["content"] = map[string]any{"en": edited}
+	env.expect(t, http.StatusOK, http.MethodPut, adminAPI(fmt.Sprintf("%s/%d", adminPostsPath, id)), token, body)
+
+	req := recv.expectOnePurge(t, "update", "post", "purge-post")
+	if req.Seen != edited {
+		t.Fatalf("the purge saw content_html %q, want the committed %q (purge before commit)", req.Seen, edited)
+	}
+}