feat(pricing): seed Tencent Hunyuan Hy3 pricing

Add Hunyuan Hy3 (released 2026-07-06, 256K context) to seed_model_pricing
under both the hunyuan-hy3 and hy3 ids, since the upstream billing id isn't
yet confirmed and one of the two should match logged usage.

Prices come from Tencent's launch-day list price (CNY 1 / 4 / 0.25 per 1M
input / output / cache-hit), converted at 1 USD ~= 7.14: 0.14 / 0.56 / 0.035.
cache_write is "0" (no published write rate). Hy3 actually uses input-length
tiered billing (<16K / 16-32K / >=32K); the single-price table holds the
lowest tier, so long-context requests are under-billed until this is revisited
against the official billing page. New model, so seed only.

src-tauri/src/database/schema.rs

@@ -1896,6 +1896,9 @@ impl Database {
                 "0.19",
                 "0",
             ),
+            // 腾讯混元 (Tencent Hunyuan)（官方 CNY 1/4/0.25 按 1 USD ≈ 7.14 折算；Hy3 阶梯计价取最低档）
+            ("hunyuan-hy3", "Hunyuan Hy3", "0.14", "0.56", "0.035", "0"),
+            ("hy3", "Hunyuan Hy3", "0.14", "0.56", "0.035", "0"),
             // MiniMax 系列
             ("minimax-m2.1", "MiniMax M2.1", "0.27", "0.95", "0.03", "0"),
             (