ship dev to main: LKG fallback + cache-key purge (prod aggregate poisoned)

[Flotapponnier]

Prod incident: the all-benchmarks aggregate cache holds 9 draft benches (poisoned during an SRH congestion window at 20:03 UTC) and every revalidation throws AllBenchmarksDraftError, preserving the poison indefinitely. No revalidation route exists; the repo pattern is a cache-key bump + deploy.

Ships dev (all staging-gated benches stay hidden on prod via REMOVED_BENCH_SLUGS) including:

• store: LKG snapshot fallback (store: last-known-good snapshot fallback (kills 'Awaiting samples' after first publish) #1188) — no more awaiting after first publish
• sec-unit fixes, QA batch, builder/oracle product pages + logos
• cache keys bumped to v36/v39/v27 (above main's v34/v37/v25 lineage) → full aggregate purge on deploy

Infra fixes already applied VPS-side: SRH_CONNECTION_POOL_SIZE=25 (root cause), worker on direct TCP.

[Flotapponnier]

Closing during a branch cleanup: this has sat open since its creation date and the work has either shipped another way or gone stale. The diff stays readable on this page if any of it is wanted again.