feat(configdb): map SiteCall to SiteCalls table (#22, #23 M3)

Bundle B1 of Audit Log #23 M3: introduces the SiteCall entity + EF mapping
for the central SiteCalls operational-state table. One row per
TrackedOperationId, mirrored from sites via best-effort telemetry then
periodic reconciliation; eventually-consistent mirror, not a dispatcher.

- src/ScadaLink.Commons/Entities/Audit/SiteCall.cs: append-once record
  with required TrackedOperationId/Channel/Target/SourceSite/Status,
  monotonic status update at the repo layer.
- src/ScadaLink.ConfigurationDatabase/Configurations/SiteCallEntityTypeConfiguration.cs:
  table SiteCalls, PK on TrackedOperationId (stored as varchar(36) via
  value conversion through the canonical 'D'-format GUID string —
  matches the wire shape used by gRPC + SQLite columns), two named
  indexes (IX_SiteCalls_Source_Created, IX_SiteCalls_Status_Updated).
- ScadaLinkDbContext: DbSet<SiteCall> SiteCalls in the existing Audit
  section, after AuditLogs.
- Tests in tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/:
  table name, PK, value-conversion shape, index presence + ordering.

src/ScadaLink.ConfigurationDatabase/ScadaLinkDbContext.cs

@@ -85,6 +85,7 @@ public class ScadaLinkDbContext : DbContext, IDataProtectionKeyContext
     // Audit
     public DbSet<AuditLogEntry> AuditLogEntries => Set<AuditLogEntry>();
     public DbSet<AuditEvent> AuditLogs => Set<AuditEvent>();
+    public DbSet<SiteCall> SiteCalls => Set<SiteCall>();
 
     // Data Protection Keys (for shared ASP.NET Data Protection across nodes)
     public DbSet<DataProtectionKey> DataProtectionKeys => Set<DataProtectionKey>();