Phase 15: add GET /packs/store endpoint

Returns all pack definitions with id, name, description, cost_coins, and
total_cards so the dashboard can render a purchasable pack store without
needing to know the definitions at compile time.

Two new tests: store listing shape and buy-then-open round-trip.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app.rs

@@ -152,6 +152,7 @@ pub async fn build(pool: Pool, cfg: Config) -> Result<Router> {
             get(routes::upgrades::get_chemistry_styles),
         )
         .route("/packs", get(routes::packs::get_packs))
+        .route("/packs/store", get(routes::packs::get_pack_store))
         .route("/packs/history", get(routes::packs::get_pack_history))
         .route("/packs/buy", post(routes::packs::post_buy_pack))
         .route("/packs/open/:pack_id", post(routes::packs::post_open_pack))