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/Migrations/20260523201754_AddAuditLogSourceNode.cs

@@ -30,6 +30,7 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                 name: "SourceNode",
                 table: "AuditLog",
                 type: "varchar(64)",
+                unicode: false,
                 maxLength: 64,
                 nullable: true);