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/db.py

@@ -1,3 +1,4 @@
+from sqlalchemy import text
 from sqlmodel import SQLModel, Session, create_engine
 
 from app.config import settings
@@ -9,6 +10,20 @@ def init_db():
     SQLModel.metadata.create_all(engine)
 
 
+def run_migrations():
+    """Run idempotent schema migrations for columns added after initial create."""
+    with engine.connect() as conn:
+        try:
+            conn.execute(
+                text(
+                    "ALTER TABLE transaction ADD COLUMN deferred_to_next_cycle BOOLEAN NOT NULL DEFAULT 0"
+                )
+            )
+            conn.commit()
+        except Exception:
+            conn.rollback()
+
+
 def get_session():
     with Session(engine) as session:
         yield session