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

frontend/src/types/index.ts

@@ -531,7 +531,7 @@ export interface UpdateGroupRequest {
 // ==================== Account & Proxy Types ====================
 
 export type AccountPlatform = 'anthropic' | 'openai' | 'gemini' | 'antigravity' | 'sora'
-export type AccountType = 'oauth' | 'setup-token' | 'apikey' | 'upstream' | 'bedrock' | 'bedrock-apikey'
+export type AccountType = 'oauth' | 'setup-token' | 'apikey' | 'upstream' | 'bedrock'
 export type OAuthAddMethod = 'oauth' | 'setup-token'
 export type ProxyProtocol = 'http' | 'https' | 'socks5' | 'socks5h'