Phase 3 sub-commit 1: ReactiveObject (single-entity twin)

ReactiveObject mirrors ReactiveListModel for a single entity. Loads via
GET <baseUrl><source>, stays in sync via Mercure SSE on `topic`, and
exposes the entity's JSON keys on a `data` QQmlPropertyMap so QML reads
them as `obj.data.title` with bindings that re-evaluate on change.

Properties:
  - source / topic / baseUrl / token (configuration)
  - data (QQmlPropertyMap*)            — entity fields
  - ready                              — initial fetch finished
  - exists                             — entity present (false on 404 / delete)
  - pending                            — at least one optimistic mutation in flight
  - error

invoke(method, path, body, optimistic) is identical in shape to
ReactiveListModel.invoke(): apply optimistic to `data`, send the
request with an Idempotency-Key, clear `pending` on the matching
Mercure echo, roll back on 4xx/5xx or 10s timeout. The rollback
restores backed-up values and removes keys we added optimistically.

Wired into the QML module; the skeleton builds clean. Used by Phase 3
sub-commit 3's todo edit form.

Includes the merged CI trigger change (workflow now runs on `main`
branch only, not `dev` — keeps Gitea-runner pressure low while we're
iterating on dev).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

framework/qml/CMakeLists.txt

@@ -33,6 +33,8 @@ qt_add_qml_module(php_qml_bridge
         src/MercureClient.cpp
         src/ReactiveListModel.h
         src/ReactiveListModel.cpp
+        src/ReactiveObject.h
+        src/ReactiveObject.cpp
     QML_FILES
         qml/RestClient.qml
         qml/AppShell.qml