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.
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