fix(pricing): add missing grok-4.5 pricing row to seed

grok-4.5 is the default model of the xAI presets but had no pricing
row, so usage dashboards recorded zero cost for it. Official list
price: $2 input / $6 output / $0.50 cached read per 1M tokens.
Seed-only change: the startup INSERT OR IGNORE backfills existing
databases; no repair entry needed for a missing row.

src-tauri/src/database/schema.rs

@@ -2232,6 +2232,7 @@ impl Database {
             ("qwq-32b", "QwQ 32B", "0.20", "0.60", "0", "0"),
             ("qwen3-32b", "Qwen3 32B", "0.16", "0.64", "0", "0"),
             // Grok 系列 (xAI)
+            ("grok-4.5", "Grok 4.5", "2", "6", "0.50", "0"),
             ("grok-4.3", "Grok 4.3", "1.25", "2.50", "0.20", "0"),
             (
                 "grok-4.20-0309-reasoning",