A ['featured', expiry] tag is self-asserted, so anyone could mint one and buy
top placement (the badge verified it, ordering didn't).
- New src/lib/featuredVerification.ts resolves the provider pubkey (cached) and
batch-checks provider-authored kind-9735 zap receipts for the featured coords.
- useTours / useSkateEvents / useUpcomingSkateEvents / useLabProfiles now rank
only receipts-backed featured items first.
- Availability fallback: if the provider can't be resolved or the query fails,
the self-asserted ordering is kept, so a provider outage never blanks paid
placement.
