refactor: merge bedrock-apikey into bedrock with auth_mode credential

Consolidate two separate channel types (bedrock + bedrock-apikey) into
a single "AWS Bedrock" channel. Authentication mode is now distinguished
by credentials.auth_mode ("sigv4" | "apikey") instead of separate types.

Backend:
- Remove AccountTypeBedrockAPIKey constant
- IsBedrock() simplified; IsBedrockAPIKey() checks auth_mode
- Add IsAPIKeyOrBedrock() helper to eliminate repeated type checks
- Extend pool mode, quota scheduling, and billing to bedrock
- Add RetryableOnSameAccount to handleBedrockUpstreamErrors
- Add "bedrock" scope to Beta Policy for independent control

Frontend:
- Merge two buttons into one "AWS Bedrock" with auth mode radio
- Badge displays "Anthropic | AWS"
- Pool mode and quota limit UI available for bedrock
- Quota display in account list (usage bars, capacity badges, reset)
- Remove all bedrock-apikey type references

backend/internal/handler/dto/mappers.go

@@ -264,8 +264,8 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 		}
 	}
 
-	// 提取 API Key 账号配额限制（仅 apikey 类型有效）
-	if a.Type == service.AccountTypeAPIKey {
+	// 提取账号配额限制（apikey / bedrock 类型有效）
+	if a.IsAPIKeyOrBedrock() {
 		if limit := a.GetQuotaLimit(); limit > 0 {
 			out.QuotaLimit = &limit
 			used := a.GetQuotaUsed()