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/Controllers/Api/AllocationController.php

@@ -59,9 +59,51 @@ class AllocationController extends Controller
 
         $allocations = $query->get();
 
+        // Compute allocation_indicator for each allocation based on project totals
+        $allocations->each(function ($allocation) {
+            $allocation->allocation_indicator = $this->computeAllocationIndicator(
+                $allocation->project_id,
+                $allocation->month
+            );
+        });
+
         return $this->wrapResource(AllocationResource::collection($allocations));
     }
 
+    /**
+     * Compute allocation indicator based on project totals.
+     */
+    private function computeAllocationIndicator(string $projectId, string $month): string
+    {
+        // Convert month to date format if needed
+        $monthDate = strlen($month) === 7 ? $month . '-01' : $month;
+
+        // Get total allocated for this project in this month
+        $totalAllocated = Allocation::where('project_id', $projectId)
+            ->where('month', $monthDate)
+            ->sum('allocated_hours');
+
+        // Get project approved estimate
+        $project = \App\Models\Project::find($projectId);
+        $approvedEstimate = $project?->approved_estimate;
+
+        // Handle no estimate
+        if (! $approvedEstimate || $approvedEstimate <= 0) {
+            return 'gray';
+        }
+
+        $percentage = ($totalAllocated / $approvedEstimate) * 100;
+
+        // Check in correct order: over first, then at capacity, then under
+        if ($percentage > 100) {
+            return 'red';
+        } elseif ($percentage >= 100) {
+            return 'green';
+        } else {
+            return 'yellow';
+        }
+    }
+
     /**
      * Create a new allocation
      *
@@ -89,7 +131,7 @@ class AllocationController extends Controller
     {
         $validator = Validator::make($request->all(), [
             'project_id' => 'required|uuid|exists:projects,id',
-            'team_member_id' => 'required|uuid|exists:team_members,id',
+            'team_member_id' => 'nullable|uuid|exists:team_members,id',
             'month' => 'required|date_format:Y-m',
             'allocated_hours' => 'required|numeric|min:0',
         ]);
@@ -101,12 +143,17 @@ class AllocationController extends Controller
             ], 422);
         }
 
-        // Validate against capacity and approved estimate
-        $capacityValidation = $this->validationService->validateCapacity(
-            $request->input('team_member_id'),
-            $request->input('month'),
-            (float) $request->input('allocated_hours')
-        );
+        // Validate against capacity and approved estimate (skip for untracked)
+        $teamMemberId = $request->input('team_member_id');
+        $capacityValidation = ['valid' => true, 'warning' => null, 'utilization' => 0];
+        
+        if ($teamMemberId) {
+            $capacityValidation = $this->validationService->validateCapacity(
+                $teamMemberId,
+                $request->input('month'),
+                (float) $request->input('allocated_hours')
+            );
+        }
 
         $estimateValidation = $this->validationService->validateApprovedEstimate(
             $request->input('project_id'),