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/Controllers/SummaryController.cs

@@ -13,12 +13,19 @@ namespace Budget.Api.Controllers;
 [Authorize]
 public class SummaryController(AppDbContext db, BudgetAuthorizationService authz) : ControllerBase
 {
-    private string UserId => User.FindFirst("sub")!.Value;
+    private IActionResult? TryGetUserId(out string userId)
+    {
+        var id = User.GetUserId();
+        if (id is null) { userId = string.Empty; return Unauthorized(); }
+        userId = id;
+        return null;
+    }
 
     [HttpGet]
     public async Task<IActionResult> Get(Guid budgetId)
     {
-        if (!await authz.CanReadAsync(budgetId, UserId)) return Forbid();
+        if (TryGetUserId(out var userId) is { } err) return err;
+        if (!await authz.CanReadAsync(budgetId, userId)) return Forbid();
 
         var budget = await db.Budgets.FindAsync(budgetId);
         if (budget is null) return NotFound();
@@ -70,7 +77,8 @@ public class SummaryController(AppDbContext db, BudgetAuthorizationService authz
     [HttpPut("tax-rate")]
     public async Task<IActionResult> UpdateTaxRate(Guid budgetId, [FromBody] UpdateTaxRateRequest req)
     {
-        if (!await authz.CanWriteAsync(budgetId, UserId)) return Forbid();
+        if (TryGetUserId(out var userId) is { } err) return err;
+        if (!await authz.CanWriteAsync(budgetId, userId)) return Forbid();
         var budget = await db.Budgets.FindAsync(budgetId);
         if (budget is null) return NotFound();
         budget.EffectiveTaxRate = req.EffectiveTaxRate;