feat(gateway): 计费规则可后台配置 —— token→积分汇率(DB) + 每模型积分权重

让「扣费按规则扣」的规则真正可后台热调（此前汇率是 env、积分权重无接口）：
- sundynix_setting KV 表 + GetSetting/SetSetting；TokensPerCredit 改为
  DB 设置优先 → env 回退 → 1000，SaveUsageEvent 按 ctx 读，改完即对后续任务生效。
- Pricing.credit_weight 纳入 UpsertPricing + ListPricing/SavePricing API。
- GET/PUT /admin/billing-config（token→积分汇率）。

live 验证：UI 存汇率=500 → billing-config/DB=500 → 新任务 credits_micro=214000
=107tok/500×1e6，规则→扣费联动精确成立。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-gateway/internal/store/pgsql.go

@@ -66,7 +66,7 @@ func OpenPostgres(dsn string) *Postgres {
 	migrateLegacyIntIDs(db)
 	migrateDocLinkToID(db)
 
-	if err := db.AutoMigrate(&User{}, &Task{}, &Eval{}, &LLMModel{}, &KB{}, &Doc{}, &Agent{}, &DocLink{}, &Pricing{}, &Prompt{}, &AuditLog{}, &GuardrailEvent{}, &Tenant{}, &TenantMember{}, &UsageEvent{}, &CreditLedger{}, &UsageRollup{}); err != nil {
+	if err := db.AutoMigrate(&User{}, &Task{}, &Eval{}, &LLMModel{}, &KB{}, &Doc{}, &Agent{}, &DocLink{}, &Pricing{}, &Prompt{}, &AuditLog{}, &GuardrailEvent{}, &Tenant{}, &TenantMember{}, &UsageEvent{}, &CreditLedger{}, &UsageRollup{}, &Setting{}); err != nil {
 		log.Printf("[store] postgres AutoMigrate 失败，降级运行: %v", err)
 		return &Postgres{}
 	}