Previous attempt blacklisted unstartable backfill source_contexts but
swept all real published flows out too. Inverted to a positive
allowlist: only flows whose _key is in STARTABLE_FLOW_KEYS reach the
Hubs / Assistant.
Today the canonical pr_to_po_def is the only runtime-startable flow in
the tenant. New entries are added by hand once we've verified a flow
actually starts via /api/runtime/transactions. Cleaner than chasing
source_context strings.
shad
e70944b6fa