perf: paginate wakatime/sync users query (#3161)

[vedant7007]

Summary

`/api/wakatime/sync` did one unbounded `.select()` and pulled every user with a wakatime API key (plus their encrypted keys) into serverless memory in a single response. At a few thousand users that would either:

• hit Supabase's ~4.5 MB response cap,
• blow the Vercel function timeout, or
• get killed by the runtime memory limit,

and the nightly sync would silently fail.

What changed

Adopted the same pagination pattern already in use by `/api/cron/sync`:

• `PAGE_SIZE = 50`
• deterministic `.order("id")` so pagination doesn't drift
• `.range(page * PAGE_SIZE, (page + 1) * PAGE_SIZE - 1)`
• `while (hasMore)` loop that breaks out when a short page is returned

Kept the inner `Promise.allSettled` `CHUNK_SIZE = 5` for parallelism across the Wakatime API within each page — that part was already sensible, just needed to be nested inside the pagination.

What's unchanged

Decrypt-then-fetch-then-upsert logic per user is identical. No behavior change for the actual sync — only the fetch/scan strategy differs.

Test plan

• `tsc --noEmit` clean
• verified fix mirrors the sibling `/api/cron/sync` pagination line-for-line (same PAGE_SIZE, same `.order("id")` + `.range()` shape, same `if (users.length < PAGE_SIZE) hasMore = false` tail check)
• once approved: manual smoke — confirm the cron completes across a few paginated batches without truncating

Fixes #3161

[github-actions]

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[vedant7007]

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