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.Api/Program.cs

@@ -62,7 +62,7 @@ app.MapHealthChecks("/healthz", new HealthCheckOptions
         [HealthStatus.Degraded] = StatusCodes.Status200OK,
         [HealthStatus.Unhealthy] = StatusCodes.Status503ServiceUnavailable,
     }
-});
+}).RequireAuthorization();
 
 app.MapFallbackToFile("index.html");