fix(backfill): anchor nightly backfill to a fixed wall clock via cron

.env.example

@@ -30,6 +30,6 @@ PR_RECONCILE_INTERVAL_MS=3600000
 PR_RECONCILE_WINDOW_DAYS=45
 
 # Nightly full repo backfill (coarse safety net; heavier — set false to disable).
+# Runs at 12:10am America/Chicago (hardcoded cron); window in days below.
 NIGHTLY_BACKFILL_ENABLED=true
-NIGHTLY_BACKFILL_INTERVAL_MS=86400000
-NIGHTLY_BACKFILL_DAYS=40
+NIGHTLY_BACKFILL_DAYS=10

packages/das/src/webhook/repo-backfill-schedule.service.ts

@@ -1,9 +1,5 @@
-import {
-  Injectable,
-  Logger,
-  OnModuleDestroy,
-  OnModuleInit,
-} from "@nestjs/common";
+import { Injectable, Logger, OnModuleInit } from "@nestjs/common";
+import { Cron } from "@nestjs/schedule";
 import { InjectQueue } from "@nestjs/bullmq";
 import { InjectRepository } from "@nestjs/typeorm";
 import { Queue } from "bullmq";
@@ -22,19 +18,17 @@ import {
 // Heavier than the reconcile sweep (re-touches every PR in the window), so it
 // runs daily and can be disabled on critical infra via env.
 const BACKFILL_ENABLED = process.env.NIGHTLY_BACKFILL_ENABLED !== "false";
-const BACKFILL_INTERVAL_MS = Number(
-  process.env.NIGHTLY_BACKFILL_INTERVAL_MS ?? 24 * 60 * 60 * 1000, // daily
-);
+// 12:10am America/Chicago. Anchored to the wall clock (not boot) so redeploys
+// don't move the window; timeZone keeps it at local midnight across CST/CDT.
+const BACKFILL_CRON = "10 0 * * *";
+const BACKFILL_TZ = "America/Chicago";
 const BACKFILL_DAYS = Number(
   process.env.NIGHTLY_BACKFILL_DAYS ?? DEFAULT_BACKFILL_DAYS,
 );
 
 @Injectable()
-export class RepoBackfillScheduleService
-  implements OnModuleInit, OnModuleDestroy
-{
+export class RepoBackfillScheduleService implements OnModuleInit {
   private readonly logger = new Logger(RepoBackfillScheduleService.name);
-  private timer: NodeJS.Timeout | null = null;
 
   constructor(
     @InjectRepository(Repo)
@@ -50,19 +44,16 @@ export class RepoBackfillScheduleService
       );
       return;
     }
-    // Unlike the reconcile sweep, don't run at startup — a deploy already
-    // implies fresh data, and this is the heavy job. Start on the interval.
-    this.timer = setInterval(
-      () => void this.backfillAll(),
-      BACKFILL_INTERVAL_MS,
+    this.logger.log(
+      `Nightly repo backfill scheduled '${BACKFILL_CRON}' (${BACKFILL_TZ})`,
     );
   }
 
-  onModuleDestroy(): void {
-    if (this.timer) clearInterval(this.timer);
-  }
-
+  // No run-at-startup (a deploy already implies fresh data, and this is heavy);
+  // the static per-repo jobId dedupes a tick that overlaps a draining run.
+  @Cron(BACKFILL_CRON, { name: "nightly-backfill", timeZone: BACKFILL_TZ })
   private async backfillAll(): Promise<void> {
+    if (!BACKFILL_ENABLED) return;
     try {
       const repos = await this.repoRepo.find({
         where: { registered: true },