Seed historical net worth snapshots and trigger live price sync on demo startup

- seed.py: adds 30 weekly NetWorthSnapshot rows (Sep 2025 → Apr 2026) so the
  Net Worth chart has full history on first boot, not just today's value
- main.py: fires price_sync_job and fx_sync_job in the background immediately
  after the scheduler starts in demo mode, so portfolio valuations are live
  from the moment the container becomes healthy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -51,6 +51,15 @@ async def lifespan(app: FastAPI):
     from app.workers.scheduler import start_scheduler, stop_scheduler
     await start_scheduler()
 
+    # Demo: sync live prices and FX rates immediately so portfolio values are fresh
+    if settings.is_demo:
+        import asyncio
+        from app.workers.price_sync import price_sync_job
+        from app.workers.fx_sync import fx_sync_job
+        asyncio.ensure_future(price_sync_job())
+        asyncio.ensure_future(fx_sync_job())
+        logger.info("demo_background_sync_queued")
+
     logger.info("startup_complete", env=settings.environment)
     yield