Add deferred transactions, revamp budget projections and UI

Adds deferred_to_next_cycle flag to transactions for billing cycle
bleed-over handling. Overhauls budget projection engine and refreshes
Budget page with improved monthly detail and transaction columns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -5,13 +5,14 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from app.api.v1.router import api_router
 from app.config import settings
-from app.db import init_db
+from app.db import init_db, run_migrations
 from app.seed import seed_db
 
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     init_db()
+    run_migrations()
     seed_db()
     yield