chore(db): align SourceNode unicode metadata + document partition-aligned index recipe

Tidies flagged by code review on the T6/T7/T8 migration bundle:

- Add `.IsUnicode(false)` to the three SourceNode EF property mappings to
  match every other ASCII varchar column on the same entities. Physical
  column was already `varchar(64)` because `HasColumnType` wins, but the EF
  model metadata flag was inconsistent.
- Add `unicode: false` to the three AddColumn<string> calls in the migrations
  + their Designer snapshots so the historical snapshots match the model.
- Update the model snapshot to carry IsUnicode(false) on each SourceNode entry.
- Document the SELECT-list invariant on SiteCallAuditRepository.QueryAsync:
  EF Core's FromSqlInterpolated requires every entity-tracked column in the
  result set, so future SiteCall columns must extend the list too.
- Amend plan Task 6 Step 2 to document the partition-aligned raw-SQL index
  recipe and the staging-table sync requirement.

src/ScadaLink.ConfigurationDatabase/Configurations/AuditLogEntityTypeConfiguration.cs

@@ -76,7 +76,8 @@ public class AuditLogEntityTypeConfiguration : IEntityTypeConfiguration<AuditEve
         // produced before this feature shipped. ASCII — varchar(64), no unicode.
         builder.Property(e => e.SourceNode)
             .HasColumnType("varchar(64)")
-            .HasMaxLength(64);
+            .HasMaxLength(64)
+            .IsUnicode(false);
 
         // Bounded unicode message column.
         builder.Property(e => e.ErrorMessage)

src/ScadaLink.ConfigurationDatabase/Configurations/NotificationOutboxConfiguration.cs

@@ -54,7 +54,8 @@ public class NotificationOutboxConfiguration : IEntityTypeConfiguration<Notifica
         // echoed onto NotifyDeliver audit rows (#23) for cross-row correlation.
         builder.Property(n => n.SourceNode)
             .HasColumnType("varchar(64)")
-            .HasMaxLength(64);
+            .HasMaxLength(64)
+            .IsUnicode(false);
 
         // OriginExecutionId (Audit Log #23): nullable uniqueidentifier carried from the
         // site so the dispatcher can echo it onto NotifyDeliver audit rows. No index —

src/ScadaLink.ConfigurationDatabase/Configurations/SiteCallEntityTypeConfiguration.cs

@@ -66,7 +66,8 @@ public class SiteCallEntityTypeConfiguration : IEntityTypeConfiguration<SiteCall
         // per-site, not per-node, on this table.
         builder.Property(s => s.SourceNode)
             .HasColumnType("varchar(64)")
-            .HasMaxLength(64);
+            .HasMaxLength(64)
+            .IsUnicode(false);
 
         // Indexes — names locked for reconciliation/migration discoverability.
         // Source_Created backs "calls from this site" (Central UI Site Calls page,