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/admin/account_handler.go

@@ -97,7 +97,7 @@ type CreateAccountRequest struct {
 	Name                    string         `json:"name" binding:"required"`
 	Notes                   *string        `json:"notes"`
 	Platform                string         `json:"platform" binding:"required"`
-	Type                    string         `json:"type" binding:"required,oneof=oauth setup-token apikey upstream bedrock bedrock-apikey"`
+	Type                    string         `json:"type" binding:"required,oneof=oauth setup-token apikey upstream bedrock"`
 	Credentials             map[string]any `json:"credentials" binding:"required"`
 	Extra                   map[string]any `json:"extra"`
 	ProxyID                 *int64         `json:"proxy_id"`
@@ -116,7 +116,7 @@ type CreateAccountRequest struct {
 type UpdateAccountRequest struct {
 	Name                    string         `json:"name"`
 	Notes                   *string        `json:"notes"`
-	Type                    string         `json:"type" binding:"omitempty,oneof=oauth setup-token apikey upstream bedrock bedrock-apikey"`
+	Type                    string         `json:"type" binding:"omitempty,oneof=oauth setup-token apikey upstream bedrock"`
 	Credentials             map[string]any `json:"credentials"`
 	Extra                   map[string]any `json:"extra"`
 	ProxyID                 *int64         `json:"proxy_id"`