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/Configurations/SiteCallEntityTypeConfiguration.cs

@@ -0,0 +1,75 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.Commons.Types;
+
+namespace ScadaLink.ConfigurationDatabase.Configurations;
+
+/// <summary>
+/// Maps the <see cref="SiteCall"/> record to the central <c>SiteCalls</c> table
+/// (Site Call Audit #22, Audit Log #23 M3 Bundle B). Operational state — NOT audit —
+/// so the table is non-partitioned, standard <c>[PRIMARY]</c> filegroup, no DB-role
+/// restriction. Two named indexes back the Central UI's "from this site" and
+/// "in this status" queries.
+/// </summary>
+public class SiteCallEntityTypeConfiguration : IEntityTypeConfiguration<SiteCall>
+{
+    public void Configure(EntityTypeBuilder<SiteCall> builder)
+    {
+        builder.ToTable("SiteCalls");
+
+        // PK is the strong-typed TrackedOperationId. Stored as varchar(36) by converting
+        // through the canonical "D"-format GUID string. Going through the string surface
+        // (rather than uniqueidentifier) keeps the column shape identical to how the id
+        // is serialised on the wire (gRPC strings, SQLite TEXT on the site) — one
+        // consistent format everywhere makes operational debugging far easier than
+        // mixing a uniqueidentifier central column with TEXT site columns.
+        builder.HasKey(s => s.TrackedOperationId);
+
+        builder.Property(s => s.TrackedOperationId)
+            .HasConversion(
+                id => id.Value.ToString("D"),
+                s => new TrackedOperationId(Guid.Parse(s)))
+            .HasMaxLength(36)
+            .IsUnicode(false)
+            .IsRequired();
+
+        // Enum-as-string columns: bounded varchar, ASCII.
+        builder.Property(s => s.Channel)
+            .HasMaxLength(32)
+            .IsUnicode(false)
+            .IsRequired();
+
+        builder.Property(s => s.Status)
+            .HasMaxLength(32)
+            .IsUnicode(false)
+            .IsRequired();
+
+        builder.Property(s => s.SourceSite)
+            .HasMaxLength(64)
+            .IsUnicode(false)
+            .IsRequired();
+
+        builder.Property(s => s.Target)
+            .HasMaxLength(256)
+            .IsUnicode(false)
+            .IsRequired();
+
+        // Bounded unicode message column.
+        builder.Property(s => s.LastError)
+            .HasMaxLength(1024);
+
+        // Indexes — names locked for reconciliation/migration discoverability.
+        // Source_Created backs "calls from this site" (Central UI Site Calls page,
+        // filter by SourceSite, newest first).
+        builder.HasIndex(s => new { s.SourceSite, s.CreatedAtUtc })
+            .IsDescending(false, true)
+            .HasDatabaseName("IX_SiteCalls_Source_Created");
+
+        // Status_Updated backs "calls in this status" (e.g. parked rows awaiting
+        // operator action, newest UpdatedAtUtc first).
+        builder.HasIndex(s => new { s.Status, s.UpdatedAtUtc })
+            .IsDescending(false, true)
+            .HasDatabaseName("IX_SiteCalls_Status_Updated");
+    }
+}

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>();