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/allocation/index.ts

@@ -0,0 +1,10 @@
+import { allocationAssignmentProcedures } from "./assignment-procedures.js";
+import { allocationDemandProcedures } from "./demand.js";
+import { allocationReadProcedures } from "./read.js";
+import { createTRPCRouter } from "../../trpc.js";
+
+export const allocationRouter = createTRPCRouter({
+  ...allocationReadProcedures,
+  ...allocationDemandProcedures,
+  ...allocationAssignmentProcedures,
+});