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/service/setting_service.go

@@ -1278,7 +1278,7 @@ func (s *SettingService) SetBetaPolicySettings(ctx context.Context, settings *Be
 		BetaPolicyActionPass: true, BetaPolicyActionFilter: true, BetaPolicyActionBlock: true,
 	}
 	validScopes := map[string]bool{
-		BetaPolicyScopeAll: true, BetaPolicyScopeOAuth: true, BetaPolicyScopeAPIKey: true,
+		BetaPolicyScopeAll: true, BetaPolicyScopeOAuth: true, BetaPolicyScopeAPIKey: true, BetaPolicyScopeBedrock: true,
 	}
 
 	for i, rule := range settings.Rules {