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/domain/constants.go

@@ -31,8 +31,7 @@ const (
 	AccountTypeSetupToken    = "setup-token"    // Setup Token类型账号（inference only scope）
 	AccountTypeAPIKey        = "apikey"         // API Key类型账号
 	AccountTypeUpstream      = "upstream"       // 上游透传类型账号（通过 Base URL + API Key 连接上游）
-	AccountTypeBedrock       = "bedrock"        // AWS Bedrock 类型账号（通过 SigV4 签名连接 Bedrock）
-	AccountTypeBedrockAPIKey = "bedrock-apikey" // AWS Bedrock API Key 类型账号（通过 Bearer Token 连接 Bedrock）
+	AccountTypeBedrock = "bedrock" // AWS Bedrock 类型账号（通过 SigV4 签名或 API Key 连接 Bedrock，由 credentials.auth_mode 区分）
 )
 
 // Redeem type constants