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/Repositories/SiteCallAuditRepository.cs

@@ -171,6 +171,11 @@ WHERE TrackedOperationId = {idText}
         // and compose with the keyset cursor, so a StuckOnly page is honest:
         // never under-filled with a non-null next cursor. Mirrors how
         // NotificationOutboxRepository.QueryAsync applies NotificationOutboxFilter.StuckCutoff.
+        //
+        // SELECT-list maintenance: EF Core's FromSqlInterpolated requires every
+        // entity-tracked column to appear in the result set. Adding a new column
+        // to the SiteCall entity means extending the list below too — otherwise
+        // every read trips "The required column 'X' was not present" at runtime.
         FormattableString sql = $@"
 SELECT TOP ({paging.PageSize})
     TrackedOperationId, Channel, Target, SourceSite, SourceNode, Status, RetryCount,