feat(stovepipe): add Queue entity (#309)

## Why?
Implementing `process` step.  

This change creates a Queue entity to keep track of queue-level state.
This matches the spec outlined in:
https://github.com/uber/submitqueue/blob/main/doc/rfc/stovepipe/steps/process.md#queue-new-persisted-entity

## What?
Add the per-queue Queue entity that backs the process stage's
concurrency gate and backlog coalescing.

## Test Plan
-Subsequent PRs will build out functionality that we can test

Author: mnoah1

stovepipe/entity/BUILD.bazel

@@ -2,7 +2,10 @@ load("@rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "go_default_library",
-    srcs = ["request.go"],
+    srcs = [
+        "queue.go",
+        "request.go",
+    ],
     importpath = "github.com/uber/submitqueue/stovepipe/entity",
     visibility = ["//visibility:public"],
 )

stovepipe/entity/queue.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package entity
+
+// Queue holds per-queue pipeline coordination state for a named repo+ref (e.g. "monorepo/main").
+type Queue struct {
+	// ****************
+	// Immutable fields, fixed at queue entity creation
+	// ****************
+
+	// Name is the stable logical id for the queue. It should be unique within the system.
+	Name string `json:"name"`
+
+	// ****************
+	// Following fields could be changed throughout the lifecycle of the queue
+	// ****************
+
+	// LastGreenURI is the queue's last-known-good commit: the most recent head at which
+	// whole-repo validation recorded green (health degree 0). Empty until the first such outcome.
+	LastGreenURI string `json:"last_green_uri"`
+
+	// InFlightCount is the number of trunk validations admitted by process but not yet terminal.
+	InFlightCount int32 `json:"in_flight_count"`
+
+	// LatestRequestSeq is the highest request sequence accepted for this queue. Used to coalesce
+	// backlog so intermediate heads are skipped without superseding the newest head.
+	LatestRequestSeq int64 `json:"latest_request_seq"`
+
+	// Version is the version of the object. It is used for optimistic locking.
+	// Versioning starts at 1 and is incremented for each change to the object.
+	Version int32 `json:"version"`
+}