feat(v5.9.0): AI Writing Assist - slash /ai, autocompletion, AI properties

- Service app/services/ai_writing.py: 6 actions sans outils (write, summarize,
  translate, continue, autocomplete, properties) + replis deterministes offline
- Endpoints POST /api/agent/writing et /api/agent/writing/properties
- Editeur: groupe slash AI (Write/Summarize/Translate/Continue) via E.aiSlash
- Autocompletion inline AIAC (suggestion ~900ms, Tab accepte, Escape rejette)
- Database: bouton AI fill (suggestions de proprietes Status/Priority/Resume)
- Tests tests/test_ai_writing.py (+29) ; version 5.9.0 (VERSION, main.py)
- CHANGELOG + ROADMAP v5.9.0 completes

app/main.py

@@ -53,7 +53,7 @@ async def lifespan(_app: FastAPI):
     from app.services.automations import automation_scheduler
     automation_task = asyncio.create_task(automation_scheduler())
 
-    logger.info("FlowDeck v5.5.0 started on port %d", settings.app_port)
+    logger.info("FlowDeck v5.9.0 started on port %d", settings.app_port)
     try:
         yield
     finally:
@@ -71,7 +71,7 @@ async def lifespan(_app: FastAPI):
 
 app = FastAPI(
     title="FlowDeck",
-    version="5.5.0",
+    version="5.9.0",
     docs_url="/docs" if settings.log_level == "DEBUG" else None,
     redoc_url=None,
     lifespan=lifespan,