Security hardening

- Remove OwnerUserId from BudgetDto: OIDC sub of the budget owner was
  being returned to all collaborators (including View-only users)
- Remove SharedWithUserId from ShareDto: other users' internal OIDC subs
  were visible to anyone with read access to a budget
- Delete MeController: scaffolding endpoint that returned sub to the
  browser; no legitimate frontend use case
- Restrict /healthz to require authorization: prevents unauthenticated
  probing of database connectivity
- Add input validation annotations to all request DTOs: [Required],
  [MaxLength], [Range(0,0.9999)] on EffectiveTaxRate, [EmailAddress] on
  share email — [ApiController] now returns 400 instead of 500 for
  invalid input hitting DB constraints
- Replace User.FindFirst("sub")!.Value with GetUserId() extension across
  all controllers: returns 401 instead of NullReferenceException (500)
  if a token lacks a sub claim

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

src/Budget.Client/src/types/index.ts

@@ -43,7 +43,6 @@ export interface OutgoDto {
 export interface BudgetDto {
   id: string;
   name: string;
-  ownerUserId: string;
   effectiveTaxRate: number;
   createdAt: string;
   updatedAt: string;
@@ -51,7 +50,6 @@ export interface BudgetDto {
 
 export interface ShareDto {
   id: string;
-  sharedWithUserId: string | null;
   sharedWithEmail: string;
   permission: SharePermission;
   isPending: boolean;