* Add ECR auto-refreshing upstream authentication
- Add "ecr" auth type to upstream.auth config with optional region
- Cache ecr:GetAuthorizationToken results per region and refresh
shortly before expiry via the AWS SDK default credential chain
- Route type: ecr through the token cache in Server.authForURL
- Document the new type in config.example.yaml and docs/configuration.md
Fixes#276
* Collapse ecrTokens.header to a single return path
Drops the internal/server package below the goconst min-occurrences
threshold for the Authorization literal.
* Coalesce concurrent ECR token fetches with singleflight
Concurrent cache misses for the same region now share a single
GetAuthorizationToken call instead of each issuing their own, avoiding
a request burst against the ECR API at cold start and at each 12-hour
refresh. golang.org/x/sync is already a direct dependency.
* Improve ECR token refresh and region inference
* Back off failed ECR token refreshes
