fix: reduce snapshots viewer cold cache work

[emmajam]

Summary

• Remove the request-path scan that listed every object in every snapshot directory
• Use bounded R2 checks against each manifest component terminal object instead
• Cache compact preset sizing data instead of raw manifest chunk arrays
• Coalesce simultaneous cold-cache refreshes within a Worker isolate

Context

snapshots.tempoxyz.dev hit Cloudflare Error 1102 because cold cache rebuilds can exceed Worker resource limits while indexing R2 snapshot data. This keeps the public request path much lighter while still filtering obviously incomplete manifests.

Validation

• pnpm --filter tempo-snapshots-viewer check
• pnpm --filter tempo-snapshots-viewer build
• wrangler dev smoke tested / and /api/snapshots with 200 responses
• pnpm check
• pnpm check:types
• pnpm precommit

No app-specific pnpm test was run because tempo-snapshots-viewer does not define a test script.

[github-actions]

Cloudflare Deployments

App	Environment	Status	Preview
explorer	devnet	[>>] Skipped	No changes
explorer	mainnet	[>>] Skipped	No changes
explorer	testnet	[>>] Skipped	No changes
fee-payer	devnet	[>>] Skipped	No changes
fee-payer	moderato	[>>] Skipped	No changes
fee-payer	privy	[>>] Skipped	No changes
og	-	[>>] Skipped	No changes
perf	-	[>>] Skipped	No changes
tempo-snapshots-viewer	-	[OK] Deployed	View Preview
tokenlist	-	[>>] Skipped	No changes