Atomic vote counting via Postgres RPC (fix TOCTOU race)

The current vote endpoint reads vote_count then writes vote_count + 1 — a classic read-modify-write race. Under concurrent load two requests can both read the same number and both write the same increment, losing votes. Replace with a single atomic UPDATE feature_requests SET vote_count = vote_count + 1 WHERE id = $1 RETURNING vote_count called via an RPC.