refactor(google-health): read and write backfill progress through one schema

The backfillProgress column was read by four hand-rolled walkers over
untyped records and written inline in the sync core. One module now owns
the column: a Zod schema reads it, lenient per entry so a malformed cursor
or end is dropped on its own, and one function writes it. The sync core
keeps only the bookkeeping of a running backfill.

src/app/api/google-health/status/route.ts

@@ -3,7 +3,7 @@ import { apiHandler, requireAuth } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
 import { apiSuccess } from "@/lib/api-response";
 import { loadOneConnectionHealth } from "@/lib/integrations/connection-health-load";
-import { googleHealthBackfillEndedList } from "@/lib/google-health/sync-core";
+import { googleHealthBackfillEndedList } from "@/lib/google-health/backfill-progress";
 
 /**
  * Get Google Health connection status for the current user (v1.26.0).

src/app/api/integrations/status/route.ts

@@ -55,7 +55,7 @@ import { loadConnectionHealth } from "@/lib/integrations/connection-health-load"
 import {
   googleHealthBackfillEndedList,
   type GoogleHealthBackfillEndedEntry,
-} from "@/lib/google-health/sync-core";
+} from "@/lib/google-health/backfill-progress";
 import type { ConnectionHealth } from "@/lib/integrations/connection-health";
 
 export const dynamic = "force-dynamic";