refactor(measurements): drop the write-only aggregation columns (migration 0272)

Migration 0263 added three columns beside the Apple Health `stats:*`
authority ladder. Only `aggregation_provenance` is ever read back: the
external-measurement reconciler selects it and refuses a lower-authority write
against a higher-authority canonical row, and the cumulative drain reads it
too. That column stays.

`aggregation_contributor_count` and `aggregation_selected_source_hash` are
written by three call sites and read by none. No select, no raw SQL, no export,
no backup, no API contract, no repair path. The import-time selection keeps its
deterministic tiebreak, which compares an in-memory map key computed per flush
rather than a value read back from the table.

Both are plain nullable scalars with no index, unique, check or foreign key, so
the drop is a catalog-only operation that touches no measured value.

src/app/api/measurements/batch/route.ts

@@ -544,8 +544,6 @@ async function postBatch(request: NextRequest): Promise<Response> {
               .rhythmClassification as Prisma.MeasurementUncheckedCreateInput["rhythmClassification"],
             deviceType: p.row.deviceType as string | null,
             aggregationProvenance: statsRow ? "HEALTHKIT_STATISTICS" : null,
-            aggregationContributorCount: null,
-            aggregationSelectedSourceHash: null,
           },
           { exactExternalMatch: statsRow ? "update" : "duplicate" },
         ),