feat(server): wire device_id through delivery proto and v2 handlers

Add device_id field to FetchRequest, FetchWaitRequest, PeekRequest,
and AckRequest proto messages. V2 handlers now build composite
queue keys (identity_key + device_id) when device_id is provided,
enabling per-device fetch/ack scoping.

proto/qpq/v1/delivery.proto

@@ -14,17 +14,25 @@ message EnqueueRequest {
   bytes payload = 2;
   bytes channel_id = 3;
   uint32 ttl_secs = 4;
+  // Client-generated idempotency key (16 bytes, UUID v7).
+  // Server deduplicates enqueue requests with the same message_id within a TTL window.
+  bytes message_id = 5;
 }
 
 message EnqueueResponse {
   uint64 seq = 1;
   bytes delivery_proof = 2;
+  // True if this was a duplicate enqueue (message_id already seen).
+  bool duplicate = 3;
 }
 
 message FetchRequest {
   bytes recipient_key = 1;
   bytes channel_id = 2;
   uint32 limit = 3;
+  // Device ID for multi-device scoping. When set, the server builds
+  // a composite queue key (identity_key + device_id). Empty = bare key.
+  bytes device_id = 4;
 }
 
 message FetchResponse {
@@ -36,6 +44,7 @@ message FetchWaitRequest {
   bytes channel_id = 2;
   uint64 timeout_ms = 3;
   uint32 limit = 4;
+  bytes device_id = 5;
 }
 
 message FetchWaitResponse {
@@ -46,6 +55,7 @@ message PeekRequest {
   bytes recipient_key = 1;
   bytes channel_id = 2;
   uint32 limit = 3;
+  bytes device_id = 4;
 }
 
 message PeekResponse {
@@ -56,6 +66,7 @@ message AckRequest {
   bytes recipient_key = 1;
   bytes channel_id = 2;
   uint64 seq_up_to = 3;
+  bytes device_id = 4;
 }
 
 message AckResponse {}
@@ -65,6 +76,8 @@ message BatchEnqueueRequest {
   bytes payload = 2;
   bytes channel_id = 3;
   uint32 ttl_secs = 4;
+  // Client-generated idempotency key (16 bytes, UUID v7).
+  bytes message_id = 5;
 }
 
 message BatchEnqueueResponse {