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/Models/ProjectMonthPlan.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Concerns\HasUuids;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class ProjectMonthPlan extends Model
+{
+    use HasUuids;
+
+    protected $table = 'project_month_plans';
+
+    protected $fillable = [
+        'project_id',
+        'month',
+        'planned_hours',
+    ];
+
+    protected $casts = [
+        'month' => 'date:Y-m-01',
+        'planned_hours' => 'decimal:2',
+    ];
+
+    /**
+     * Get the project this plan belongs to.
+     */
+    public function project(): BelongsTo
+    {
+        return $this->belongsTo(Project::class);
+    }
+
+    /**
+     * Check if this plan cell is blank (unset).
+     */
+    public function isBlank(): bool
+    {
+        return $this->planned_hours === null;
+    }
+
+    /**
+     * Get planned hours or 0 for variance calculations.
+     * Blank plan is treated as 0 for allocation variance.
+     */
+    public function getPlannedHoursForVariance(): float
+    {
+        return (float) ($this->planned_hours ?? 0);
+    }
+}