fix finance balance delta recharges

backend/app/services/snapshot_service.py

@@ -1,9 +1,10 @@
-"""Snapshot diff logic."""
+"""Snapshot diff logic and balance snapshot writes."""
+from datetime import datetime, timezone
 from typing import Any, Optional
 
 from sqlalchemy.orm import Session
 
-from app.models.snapshot import UpstreamRateSnapshot
+from app.models.snapshot import UpstreamBalanceSnapshot, UpstreamRateSnapshot
 
 
 def diff_snapshots(
@@ -43,6 +44,16 @@ def diff_snapshots(
     return changes
 
 
+def write_balance_snapshot(db: Session, upstream_id: int, balance: float) -> None:
+    """Write a balance snapshot row. Call only on successful balance fetch."""
+    row = UpstreamBalanceSnapshot(
+        upstream_id=upstream_id,
+        balance=balance,
+        captured_at=datetime.now(timezone.utc),
+    )
+    db.add(row)
+
+
 def prune_snapshots(db: Session, upstream_id: int, keep: int) -> None:
     if keep <= 0:
         return