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/database/migrations/2026_02_25_222332_make_team_member_id_nullable_in_allocations.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('allocations', function (Blueprint $table) {
+            $table->uuid('team_member_id')->nullable()->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('allocations', function (Blueprint $table) {
+            $table->uuid('team_member_id')->nullable(false)->change();
+        });
+    }
+};

backend/database/migrations/2026_02_26_000001_create_project_month_plans_table.php

@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('project_month_plans', function (Blueprint $table) {
+            $table->uuid('id')->primary();
+            $table->uuid('project_id');
+            $table->date('month'); // First day of month
+            $table->decimal('planned_hours', 10, 2)->nullable(); // null = blank/unset
+            $table->timestamps();
+
+            // Unique constraint: one plan per project per month
+            $table->unique(['project_id', 'month']);
+
+            // Foreign key
+            $table->foreign('project_id')
+                ->references('id')
+                ->on('projects')
+                ->onDelete('cascade');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('project_month_plans');
+    }
+};