fix(cache): share one in-flight load across concurrent cache misses

[cryptiklemur]

Summary

• TTLCache in src/lib/db/readCache.ts gets a load(key, loader) method. Callers that miss the same key at the same time now share one pending load instead of each starting their own.
• every cached wrapper (settings, pricing, connections, raw connections, connection by id, nodes, LKGP) uses it.
• invalidate() also drops the pending load, so a caller that arrives after an invalidation starts a fresh read and never gets the old one.
• why: the nightly usage_history rollup (src/lib/usage/aggregateHistory.ts) prices thousands of groups in one Promise.all. it often runs right after a models.dev sync clears the pricing cache. every group missed, and each one loaded about 1.5 MB of pricing. that ran a 20 GB heap out of memory.

Related Issues

• none filed. seen in production as an OOM during the nightly rollup.

Validation

• Change type: DB
• Focused tests and category gates from the golden path
• npm run lint: no errors. it exits 2 only because release/v3.8.51 has 5 stale suppressions in unrelated files (openai-to-cursor.ts, SystemStorageTab.tsx, both vscode combos/route.ts, ghe-copilot.ts). the same happens on the base, and --pass-on-unpruned-suppressions exits 0.
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

check	result
new test on release/v3.8.51 without the fix	2 of 3 fail (150 and 600 pricing reads, expected 3)
new test with the fix	3 of 3 pass
db-read-cache, usage-history-*, combo-cache-invalidation, combo-delete-lkgp-cleanup-12326, db-synced-model-catalog-invalidation-8728	pass
eslint + prettier on touched files	clean

Tests Added Or Updated

• tests/unit/pricing-cache-single-flight.test.ts (new)

Coverage Notes

• the new test covers 50 concurrent getCachedPricing() misses, an invalidation while a load is pending, and a full rollupUsageHistoryBeforeDate() over 200 distinct rows. the other wrappers share the same load() path.

Reviewer Notes

• a failed load is not cached. the next caller retries, same as before.
• usage-history-provider-alias-13459.test.ts fails about 1 run in 4 when run next to other DB tests. it does this on the base branch too, without this change.