feat: fiat price feed with per-feed operator control (GET /api/v1/prices)

Cached crypto→fiat price feed, refreshed by a background task and served from an
in-memory snapshot — no upstream call on the request path. Public and
feature-flagged: when FEATURE_PRICES is off, /prices is 404 and no upstream is
ever contacted.

Per-feed control, as a public server needs: PRICES_ASSETS narrows which feeds an
instance quotes (unset = all supported; an explicit list serves a subset; empty
serves none), validated fail-closed against SUPPORTED_PRICE_ASSETS. Provider
(PRICES_PROVIDER, coingecko) and currency (PRICES_CURRENCY, usd) are configurable;
the refresh interval is clamped to a provider-friendly minimum.

- infra/prices: PriceClient (feed whitelist → provider coin ids), bounded by the
  HTTP client timeout; non-finite/negative/missing quotes are skipped, not faked.
  A fetch failure keeps the last good snapshot. Drift guard: a test asserts the
  provider mapping covers exactly SUPPORTED_PRICE_ASSETS.
- main: background refresh task only when enabled and at least one feed is set.
- AppState.prices snapshot; handler 404s when disabled.
- openapi (prices tag); unit tests + L1 test over both feature-flag states.

src/api/mod.rs

@@ -6,6 +6,7 @@ pub mod health;
 pub mod middleware;
 pub mod nip05;
 pub mod openapi;
+pub mod prices;
 pub mod users;
 pub mod wallet;
 pub mod website;