Add trust features: history, trash, export, and collection sharing

Implement Notion-like upgrade phase 4: append-only page revisions with restore-as-a-new-version; recoverable bookmark/page/collection deletions whose restores never resurrect public flags or shares; owner-scoped JSON export; and explicit, revocable read-only collection sharing with owner-only mutations and no transitive third-party bookmark leaks. Add migration 009, API/docs, integration coverage, and page/collections/trash UI.

internal/handlers/unshare.go

@@ -20,6 +20,9 @@ func HandleBookmarkUnshare(w http.ResponseWriter, r *http.Request) {
 func HandleTagUnshare(w http.ResponseWriter, r *http.Request) {
 	unshare(w, r, "Tag", "TagShare", "tagId")
 }
+func HandleCollectionUnshare(w http.ResponseWriter, r *http.Request) {
+	unshare(w, r, "Collection", "CollectionShare", "collectionId")
+}
 func HandleSharedDateDelete(w http.ResponseWriter, r *http.Request) {
 	respond(w, 200, OK{true}, remove(r, "SharedDate", r.PathValue("id"), "creatorId"))
 }