feat: M8.2 Gateway Queue Adapter for SQS via api.riotpiao.com

- Unified QueueAdapter trait for concurrent dual-write operations
- GatewayQueueAdapter routes messages via api.riotpiao.com with X-Service: sqs header
- TokenProvider abstraction: StaticTokenProvider + AuthentikTokenProvider
- JWT bearer token support (from Authentik OAuth2)
- InMemoryQueueAdapter for testing
- Base64 encoding/decoding for SQS message bodies
- HTTP/REST integration (no direct gRPC complexity)
- 8 unit tests + comprehensive documentation
- Supports long-polling (ReceiveMessage), visibility timeout, DLQ

Uses standard SQS API patterns:
- SendMessage: Queue chunk for dual-write processing
- ReceiveMessage: Long-poll up to 10 messages, 20s wait
- DeleteMessage: Acknowledge on success
- ChangeMessageVisibility: Retry on failure
- SendToDLQ: After max retries

Files:
- crates/mem-cli/src/queue_adapter.rs (310 LOC)
- crates/mem-cli/src/gateway_queue_adapter.rs (530 LOC)
- tests/it_gateway_queue_adapter.rs (110 LOC)
- docs/M8.2-GATEWAY_QUEUE_ADAPTER.md (400 LOC)

Cargo.lock

@@ -2031,6 +2031,7 @@ dependencies = [
  "actix-rt",
  "actix-web",
  "anyhow",
+ "async-trait",
  "base64 0.21.7",
  "chrono",
  "clap",
@@ -2053,6 +2054,7 @@ dependencies = [
  "tokio",
  "tracing",
  "tracing-subscriber",
+ "urlencoding",
  "uuid",
 ]