Security & resource hardening: eliminate CPU/disk attack surface

Addresses production CPU spike incident. Key changes:

- Guard OTel exporter behind OTEL_EXPORTER_OTLP_ENDPOINT env var; filter
  tracing to /api paths only — unconditional export was primary suspect
- Remove /healthz endpoint entirely (unauthenticated, hit DB on every call)
- Replace KnownUserMiddleware with POST /api/users/me called once on login
  from TokenSync — eliminates unconditional DB write on every request
- Add DB indexes: (BudgetId, IsDeleted) on Incomes/Outgos, OwnerUserId on
  Budgets, SharedWithUserId and (IsPending, SharedWithEmail) on BudgetShares
- Move UseRateLimiter() before UseStaticFiles() so all requests are throttled
- Replace full-array reorder with move-by-position (id + newIndex) — bounded
  input, fewer DB writes, better API design
- Lock ForwardedHeaders to 172.20.0.0/16 subnet; fixes KnownNetworks
  deprecation warning (0 warnings in build now)
- Add AsNoTracking() to all read-only queries in Summary/Incomes/OutgosController
- FrequencyCalculator returns 0 for unknown enum values instead of throwing
- Thread.Sleep → await Task.Delay in OIDC startup loop
- AllowedHosts locked to budget.stwaddle.com

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

src/Budget.Api/appsettings.json

@@ -5,7 +5,7 @@
       "Microsoft.AspNetCore": "Warning"
     }
   },
-  "AllowedHosts": "*",
+  "AllowedHosts": "budget.stwaddle.com",
   "ConnectionStrings": {
     "DefaultConnection": ""
   },