feat(db): IngestionRun.zipsBackfilled counter

Additive migration (applied on deploy via prisma migrate deploy). Counts
packages whose provenance was backfilled during an ingestion run.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

prisma/migrations/20260723000000_ingestion_run_zips_backfilled/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable: count of packages whose provenance was backfilled during a run
+-- (opportunistic cross-channel provenance backfill). Additive, non-null with a
+-- default of 0 — no data change for existing rows.
+ALTER TABLE "ingestion_runs"
+  ADD COLUMN "zipsBackfilled" INTEGER NOT NULL DEFAULT 0;

prisma/schema.prisma

@@ -569,6 +569,7 @@ model IngestionRun {
   zipsFound       Int             @default(0)
   zipsDuplicate   Int             @default(0)
   zipsIngested    Int             @default(0)
+  zipsBackfilled  Int             @default(0)
   errorMessage    String?
 
   // Live activity tracking — written by worker in real-time