fix: ensure DEV mode warning appears in Docker logs on startup

- Add FastAPI @app.on_event("startup") handler to display warning
- Previously only appeared when running directly (not via uvicorn)
- Add DEPLOYMENT_MODE and PRESERVE_DEV_DATA to docker-compose.yml
- Update CHANGELOG.md with fix documentation

Fixes issue where dev mode banner wasn't visible in Docker logs
because uvicorn imports app without executing __main__ block.

api/main.py

@@ -136,6 +136,11 @@ def create_app(
     app.state.db_path = db_path
     app.state.config_path = config_path
 
+    @app.on_event("startup")
+    async def startup_event():
+        """Display DEV mode warning on startup if applicable"""
+        log_dev_mode_startup_warning()
+
     @app.post("/simulate/trigger", response_model=SimulateTriggerResponse, status_code=200)
     async def trigger_simulation(request: SimulateTriggerRequest):
         """