docs(openspec): add reporting API contract documentation

Add comprehensive API documentation for the reporting endpoint:
- Request/response structure
- View type inference (did/is/will)
- Blank vs explicit zero semantics
- Status values and error responses

Related to enhanced-allocation change.

backend/app/Http/Resources/ProjectMonthPlanResource.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Http\Resources;
+
+use Illuminate\Http\Request;
+
+class ProjectMonthPlanResource extends BaseResource
+{
+    public function toArray(Request $request): array
+    {
+        return [
+            'id' => $this->id,
+            'project_id' => $this->project_id,
+            'month' => $this->month?->format('Y-m'),
+            'planned_hours' => $this->formatDecimal($this->planned_hours),
+            'is_blank' => $this->planned_hours === null,
+            'created_at' => $this->formatDate($this->created_at),
+            'updated_at' => $this->formatDate($this->updated_at),
+        ];
+    }
+}