feat: implement MLS lifecycle and multi-device support

Phase 5.3 (MLS lifecycle):
- Add group.proto with RemoveMember, UpdateGroupMetadata, ListGroupMembers, RotateKeys RPCs
- Add GroupService domain logic with metadata and membership persistence
- Add v2 RPC handlers for all 4 group management endpoints (method IDs 410-413)
- Add SDK functions: remove_member_from_group, leave_group, rotate_group_keys, set_group_metadata, get_group_members
- Add REPL commands: /group remove, /group rename, /group rotate-keys, /group leave
- Add 5 unit tests for GroupService (metadata CRUD, membership add/list/remove)

Phase 5.1 (multi-device):
- Wire device_id through SDK fetch/ack functions (fetch_for_device, ack)
- Add /devices list|add|remove REPL commands with tab completion
- Add clear_failed_outbox to ConversationStore
- Fix missing message_id/device_id fields in SDK proto struct initializers

proto/qpq/v1/group.proto

@@ -0,0 +1,61 @@
+syntax = "proto3";
+package qpq.v1;
+
+// Group management (4 methods).
+// Method IDs: 410-413.
+
+// RemoveMember (410): Remove a member from a group.
+message RemoveMemberRequest {
+  bytes group_id = 1;
+  bytes member_identity_key = 2;
+}
+
+message RemoveMemberResponse {
+  bytes commit = 1;
+}
+
+// UpdateGroupMetadata (411): Set group name, description, avatar.
+message UpdateGroupMetadataRequest {
+  bytes group_id = 1;
+  string name = 2;
+  string description = 3;
+  bytes avatar_hash = 4;
+}
+
+message UpdateGroupMetadataResponse {
+  bool success = 1;
+}
+
+// ListGroupMembers (412): List members of a group.
+message ListGroupMembersRequest {
+  bytes group_id = 1;
+}
+
+message ListGroupMembersResponse {
+  repeated GroupMemberInfo members = 1;
+}
+
+message GroupMemberInfo {
+  bytes identity_key = 1;
+  string username = 2;
+  uint64 joined_at = 3;
+}
+
+// RotateKeys (413): Trigger key rotation (Update proposal + Commit).
+message RotateKeysRequest {
+  bytes group_id = 1;
+}
+
+message RotateKeysResponse {
+  bytes commit = 1;
+}
+
+// GroupMetadata stored server-side.
+message GroupMetadata {
+  bytes group_id = 1;
+  string name = 2;
+  string description = 3;
+  bytes avatar_hash = 4;
+  bytes creator_key = 5;
+  uint64 created_at = 6;
+}