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/AllocationResource.php

@@ -14,6 +14,7 @@ class AllocationResource extends BaseResource
             'team_member_id' => $this->team_member_id,
             'month' => $this->month?->format('Y-m'),
             'allocated_hours' => $this->formatDecimal($this->allocated_hours),
+            'allocation_indicator' => $this->allocation_indicator ?? 'gray',
             'created_at' => $this->formatDate($this->created_at),
             'updated_at' => $this->formatDate($this->updated_at),
             'project' => $this->whenLoaded('project', fn () => new ProjectResource($this->project)),