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/api/admin/settings.ts

@@ -316,7 +316,7 @@ export async function updateRectifierSettings(
 export interface BetaPolicyRule {
   beta_token: string
   action: 'pass' | 'filter' | 'block'
-  scope: 'all' | 'oauth' | 'apikey'
+  scope: 'all' | 'oauth' | 'apikey' | 'bedrock'
   error_message?: string
 }