Add Purchase Request system with board approval - #16

Summary

A standalone feature: members can request to buy something (item name, reason, link, price, quantity, needed-by date). Admin/Leader board members can approve or reject it, with an optional rejection reason.

Backend

• PurchaseRequest entity + PurchaseRequestStatus enum (Pending/Approved/Rejected), EF config, AddPurchaseRequests migration
• IPurchaseRequestService/PurchaseRequestService: submit, list-own, board queue, approve, reject — with status guards preventing re-deciding an already-decided request
• PurchaseRequestsController: create + view-own for any authenticated user; list/approve/reject gated to Admin,Leader
• Domain events + notification handlers consolidated under DomainEvents/Purchasing/ (reuses the existing shared INotificationService, no duplicate service): board is notified when a request is submitted, submitter is notified when it's approved or rejected
• 3 new NotificationType cases; DI wired in Program.cs

Frontend

• purchase-requests.service.ts (thin HttpClient wrapper)
• "Purchase Requests" page — submit form + your own requests, status tags, rejection reason shown when rejected
• "Purchase Approvals" page — board-only queue with a view/approve/reject dialog
• Routes + a "Purchasing" menu section (Approvals link hidden from non-board users; the route itself is also role-guarded)

Test plan

• Backend builds clean (dotnet build)
• Frontend builds clean (ng build), both pages compile as separate lazy chunks
• Manual: submit a request as a normal user, see it Pending
• Manual: approve one and reject another as Admin/Leader, with a rejection reason
• Manual: confirm a non-board user is redirected off /dashboard/purchase-approvals
• Manual: confirm board gets notified on submit, submitter gets notified on the decision

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