perf(sponsors): add indexes and fragment caching for sponsors page

[mroderick]

Adds two PostgreSQL indexes and per-sponsor fragment caching to the sponsors page to address the slow SponsorsController#index reports.

Database indexes

SponsorsController#index loads active sponsors ordered by updated_at, and SponsorsSearch (admin sponsors list) orders by lower(sponsors.name). Both queries previously used a sequential scan plus an explicit sort.

Local EXPLAIN ANALYZE with ~1,160 rows:

Query	Before	After
Active sponsors by updated_at	Seq Scan + Sort, ~1.45 ms	Index Scan, ~0.31 ms
Admin order by lower(name)	Seq Scan + Sort, ~2.86 ms	Index Scan, ~0.05 ms

View allocations

Scout APM showed some requests hitting ~5M allocations, 95% in view code. Profiling the sponsor partial showed CarrierWave creating uploader/version/sanitized-file objects for every sponsor on every render.

Splitting the sponsor partial into a single-sponsor partial and rendering the collection with cached: true drops warm-cache allocations for the partial by ~80% (from ~225k objects to ~44k objects locally). Cache keys are based on each sponsor's updated_at, so edits invalidate automatically.

The indexes are added concurrently with if_not_exists: true, so the migration is safe to re-run.