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/routes/api.php

@@ -5,6 +5,7 @@ use App\Http\Controllers\Api\AuthController;
 use App\Http\Controllers\Api\CapacityController;
 use App\Http\Controllers\Api\HolidayController;
 use App\Http\Controllers\Api\ProjectController;
+use App\Http\Controllers\Api\ProjectMonthPlanController;
 use App\Http\Controllers\Api\PtoController;
 use App\Http\Controllers\Api\TeamMemberController;
 use App\Http\Middleware\JwtAuth;
@@ -42,6 +43,10 @@ Route::middleware(JwtAuth::class)->group(function () {
     Route::put('projects/{project}/estimate', [ProjectController::class, 'setEstimate']);
     Route::put('projects/{project}/forecast', [ProjectController::class, 'setForecast']);
 
+    // Project Month Plans
+    Route::get('/project-month-plans', [ProjectMonthPlanController::class, 'index']);
+    Route::put('/project-month-plans/bulk', [ProjectMonthPlanController::class, 'bulkUpdate']);
+
     // Capacity
     Route::get('/capacity', [CapacityController::class, 'individual']);
     Route::get('/capacity/team', [CapacityController::class, 'team']);