refactor+feat(backend): ChargeableCharConfig por ProductType + Reactivate + Delete endpoints (PRC-001)

Part A — MedioId → ProductTypeId rename across all C# layers:
  Domain, Application, Infrastructure, API, all test projects.
  Solution was non-compilable after BD refactor (5c1675e); now compiles clean (0 errors).

Part B — PATCH /api/v1/admin/chargeable-chars/{id}/reactivate:
  ReactivateChargeableCharConfigCommand/Handler, SP guard maps 50410/50411/50412
  → ChargeableCharConfigReactivationNotAllowedException(Reason) → HTTP 409.

Part C — DELETE /api/v1/admin/chargeable-chars/{id}:
  DeleteChargeableCharConfigCommand/Handler, physical DELETE on SYSTEM_VERSIONED table.
  KeyNotFoundException → 404 via ExceptionFilter.

Tests: +30 unit tests (TDD RED→GREEN). All 1266 unit tests pass.

src/api/SIGCM2.Application/Pricing/ChargeableChars/Delete/DeleteChargeableCharConfigResponse.cs

@@ -0,0 +1,6 @@
+namespace SIGCM2.Application.Pricing.ChargeableChars.Delete;
+
+/// <summary>
+/// PRC-001 — Response for a successful delete operation.
+/// </summary>
+public sealed record DeleteChargeableCharConfigResponse(long Id);