fix: paginate weekly-digest cron query to prevent serverless timeout

[Meera2906]

Problem

The weekly-digest cron fetched all opted-in users in a single unbounded
Supabase query. With 200+ users and ~300ms per email send, the function
reliably exceeded Vercel's 60s timeout, causing partial delivery with no
retry mechanism.

Fix

Replaced the unbounded query with a paginated loop using .range():

• PAGE_SIZE = 50 rows fetched per Supabase query
• Existing BATCH_SIZE = 5 parallel email sends is preserved per page
• Each iteration stays well within the serverless timeout window
• totalFetched added to logs for observability

Fixes #1852

[github-actions]

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[Priyanshu-byte-coder]

This PR has merge conflicts with the current main branch. Please rebase or merge main into your branch to resolve the conflicts, then push the updated branch. Once conflicts are resolved, this PR will be merged.

git fetch origin main
git merge origin/main
# resolve conflicts
git push