perf: paginate discord-sync + push filters into SQL (#3163)

[vedant7007]

Summary

`/api/notifications/discord-sync` did one unbounded `.select()` and pulled every user with a discord webhook (webhook URL, timezone, mute state, last notification timestamp) into serverless memory in a single response — then serially walked each through 2-3 GitHub API roundtrips + a Supabase update. Both scaled with total user count, so once discord users cross a few hundred the outer loop can't finish within the Vercel function timeout and users at the tail get silently skipped every hour.

What changed

• Adopted the pagination pattern already used by `/api/cron/sync` (`PAGE_SIZE=50`, deterministic `.order("id")`, `.range(page50, page50+49)`, break out when a short page returns).
• Pushed the "hasn't been notified in the last 20 hours" and "not currently muted" filters into the SQL `WHERE` clause via `.or()` so we don't fetch rows we'll immediately skip. Cuts the wire payload per page down to actual candidates.
• Cached `twentyHoursAgo` and `nowIso` outside the loop.

What's unchanged

The per-user `localHour` / weekday / streak-at-risk / weekly-summary / milestone / notification-sending logic is identical to the old code path. No behavior change for notification decisions — only the fetch/scan strategy differs.

Test plan

• `tsc --noEmit` clean
• verified the `.or()` clauses match the previous in-JS filter logic (NULL or condition satisfied)
• once approved: manual smoke with a couple of test users to confirm pagination unwinds correctly on both an empty tail and a full page

Fixes #3163

[github-actions]

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[github-actions]

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[vedant7007]

starred the repo — should turn the star-required gate green on the next re-run 🌟

[github-actions]

🎉 Merged! Thanks for contributing to DevTrack.

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