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/router/index.ts

@@ -1,5 +1,5 @@
 import { createTRPCRouter } from "../trpc.js";
-import { allocationRouter } from "./allocation.js";
+import { allocationRouter } from "./allocation/index.js";
 import { assistantRouter } from "./assistant.js";
 import { auditLogRouter } from "./audit-log.js";
 import { calculationRuleRouter } from "./calculation-rules.js";