feat(pricing): seed GPT-5.6 Sol/Terra/Luna pricing

Add the three GPT-5.6 tiers to seed_model_pricing, cross-checked against
OpenAI's official pricing page and OpenRouter:

  gpt-5.6-sol    5 / 30 / 0.50 (cache read)
  gpt-5.6-terra  2.50 / 15 / 0.25
  gpt-5.6-luna   1 / 6 / 0.10

cache_write is "0": OpenAI publishes only a cached-input (read) rate and no
separate cache-write price, matching the gpt-5.x family convention. Base ids
only for now — effort-suffix variants are deferred until the upstream logging
suffix set for 5.6 is confirmed. New model, so seed only (no repair branch,
no SCHEMA_VERSION bump).

src-tauri/src/database/schema.rs

@@ -1455,6 +1455,10 @@ impl Database {
                 "0.30",
                 "3.75",
             ),
+            // GPT-5.6 系列（Sol / Terra / Luna，2026-06 发布）
+            ("gpt-5.6-sol", "GPT-5.6 Sol", "5", "30", "0.50", "0"),
+            ("gpt-5.6-terra", "GPT-5.6 Terra", "2.50", "15", "0.25", "0"),
+            ("gpt-5.6-luna", "GPT-5.6 Luna", "1", "6", "0.10", "0"),
             // GPT-5.5 系列
             ("gpt-5.5", "GPT-5.5", "5", "30", "0.50", "0"),
             ("gpt-5.5-low", "GPT-5.5", "5", "30", "0.50", "0"),