refactor(api): reorganise allocation router into allocation/ subdirectory

Moves read, assignment-procedures, assignment-mutations, and demand
procedures into allocation/ so the domain boundary is discoverable
without grep.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/api/src/__tests__/allocation-router.test.ts

@@ -1,6 +1,6 @@
 import { AllocationStatus, PermissionKey, SystemRole } from "@capakraken/shared";
 import { beforeEach, describe, expect, it, vi } from "vitest";
-import { allocationRouter } from "../router/allocation.js";
+import { allocationRouter } from "../router/allocation/index.js";
 import { emitAllocationCreated, emitAllocationDeleted, emitNotificationCreated } from "../sse/event-bus.js";
 import { checkBudgetThresholds } from "../lib/budget-alerts.js";
 import { generateAutoSuggestions } from "../lib/auto-staffing.js";