feat: implement WorkflowAdapter with namespace pass-down support

Enable X-Service: workflow routing to Temporal via ServiceAdapter.
Users can now specify namespace/domain in request payload for multi-tenant
workflow access.

Changes:
- Implement WorkflowAdapter in serviceadapter/workflow_adapter.go
  * Defines 10 workflow resources: start, describe, list, history,
    terminate, cancel, signal, query, reset, update
  * Each resource validates namespace parameter in payload
  * Forwards requests to Temporal gRPC handler

- Add GetWorkflowSpec() to define ServiceAdapter spec with:
  * Upstream: grpc://temporal:7233
  * Auth requirements per operation (execute, read, signal, query)
  * Request/response schemas for validation

- Wire WorkflowAdapter into main.go:
  * Register workflow adapter in serviceadapter registry
  * Initialize with temporal handler for gRPC forwarding

- Remove old empty WorkflowAdapter stub from adapters.go

Usage:
  curl -X POST https://api.riotpiao.com/ \
    -H 'X-Service: workflow' \
    -H 'X-Resource: start' \
    -H 'Authorization: Bearer TOKEN' \
    -d '{
      "namespace": "default",
      "workflow_id": "my-workflow",
      "workflow_type": "MyWorkflow",
      "task_queue": "default"
    }'

Namespace is required in all workflow operations and must be specified
by the client in the request payload. This enables multi-tenant support
where different teams access their own Temporal namespaces.

internal/serviceadapter/adapters.go

@@ -1,8 +1,5 @@
 package serviceadapter
 
-// WorkflowAdapter handles X-Service: workflow requests.
-type WorkflowAdapter struct{}
-
 // SQSAdapter handles X-Service: sqs requests.
 type SQSAdapter struct{}