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.Domain/Pricing/Exceptions/ChargeableCharConfigReactivationNotAllowedException.cs

@@ -0,0 +1,29 @@
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Domain.Pricing.Exceptions;
+
+/// <summary>
+/// PRC-001 — Thrown when a reactivation attempt is blocked by a guard rule.
+/// Maps to HTTP 409.
+///
+/// Reason codes:
+///   ALREADY_ACTIVE       — target row is currently active (50410)
+///   VIGENTE_EXISTS       — a different active row exists for (ProductTypeId, Symbol) (50411)
+///   POSTERIOR_ROWS_EXIST — rows with higher ValidFrom exist after the target row (50412)
+/// </summary>
+public sealed class ChargeableCharConfigReactivationNotAllowedException : DomainException
+{
+    public long Id { get; }
+
+    /// <summary>
+    /// "ALREADY_ACTIVE" | "VIGENTE_EXISTS" | "POSTERIOR_ROWS_EXIST"
+    /// </summary>
+    public string Reason { get; }
+
+    public ChargeableCharConfigReactivationNotAllowedException(long id, string reason)
+        : base($"Reactivation not allowed for config {id}: {reason}")
+    {
+        Id = id;
+        Reason = reason;
+    }
+}