demo: workers kv counter

a single global counter stored in workers kv. every visitor's click increments the same key from any colo. kv is eventually consistent globally, so a burst of clicks from different regions can briefly show stale reads. fine for counters, not for banking.

# how it works

1. page function /api/counter binds to kv namespace SITE_counter_kv
2. GET reads the global_counter key
3. POST reads, adds 1, writes it back
4. no transaction, no lock. two writers racing can overwrite each other. that's kv.

# config