feat: increase watchlist item limit and prewarm installs (STR-52)

[leo-mathurin]

Summary

• raise the supported IMDb watchlist/list ceiling to 15,000 items
• fetch adaptive 750-item GraphQL pages, including an exact-size final request at cursor boundaries
• start cache-first prewarming after configuration saves with Vercel waitUntil
• add background lifecycle, failure-isolation, pagination, and config-route coverage

Performance validation

• real 10,000-title cold scrape: 44.1s on the Vercel preview
• config response returned in 962ms while the scrape continued in the same invocation
• the first catalog request used the completed R2 cache without another IMDb fetch
• private-list prewarm failures remained background-only, with no 5xx or timeout logs

Preview: https://stremlist-backend-b4n6rbi25-lelemathrins-projects.vercel.app

Tests

• pre-commit pnpm turbo run build test
• 122 backend tests passed
• backend typecheck, lint, format check, and production build passed

Issue

STR-52

[linear]

STR-52

[vercel]

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2 Skipped Deployments

Project	Deployment	Actions	Updated
stremlist-backend	Ignored	Preview	Sep 1, 2026 2:09pm UTC
stremlist-frontend	Ignored	Preview	Sep 1, 2026 2:09pm UTC

[greptile-apps]

Greptile Summary

The PR raises the supported IMDb list size and adds cache-first background prewarming after configuration saves.

• Uses adaptive page sizes at IMDb cursor-window boundaries.
• Runs prewarms through Vercel background lifecycle support with bounded concurrency and cross-instance leases.
• Coalesces concurrent cache misses and adds pagination, lifecycle, and route coverage.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
apps/backend/src/services/imdb-scraper.ts	Adds adaptive 750-item pagination while respecting 10,000-item cursor windows and a 15,000-item ceiling.
apps/backend/src/services/watchlist-prewarm.ts	Adds bounded cache-first prewarming with per-instance coalescing and database-backed owner leases.
apps/backend/src/services/watchlist.ts	Coalesces concurrent canonical refreshes before applying caller-specific sorting and poster customization.
apps/backend/src/routes/api.ts	Schedules prewarming after successful configuration persistence without delaying the response.
supabase/migrations/20260901110000_add_watchlist_prewarm_lease.sql	Adds a bounded, service-role-only atomic lease used to coordinate prewarms across instances.

_{Reviews (5): Last reviewed commit: "fix(backend): coalesce concurrent watchl..." | Re-trigger Greptile}

[leo-mathurin]

@greptile review

[leo-mathurin]

@greptile review

[leo-mathurin]

@greptile review

[leo-mathurin]

@greptile review