feat(api): nullable deviceType on Measurement for two-axis source priority

Add an optional deviceType field to the POST /api/measurements/batch
payload schema. The iOS client maps each sample's HKDevice.model to a
canonical device class (watch | band | ring | phone | scale | other |
unknown); the server stores it on a new Measurement.device_type
column. The cross-source canonical picker (W8c) reads this column to
break Apple-Watch-vs-iPhone ties when the same source contributed
multiple devices for the same day.

Backward-compatible end-to-end:
  - Schema field is optional; legacy iOS builds keep working unchanged.
  - DB column is nullable; the migration is an additive ALTER TABLE
    (Migration 0051, next free per v1.4.25 handoff section 4).
  - The picker treats null as 'unknown' and only uses it as a
    tiebreaker when a ranked device-type coexists in the bucket; with
    a single source + null deviceType the row passes through as today.

OpenAPI: deviceType is documented on AppleHealthBatchEntry with
guidance on the HKDevice.model mapping. Batch endpoint description
gains a W8c note. iOS-Claude can lock the field name + enum from the
spec without re-reading source.

Tests: 45/45 source-priority tests pass; the batch route's existing
integration tests are untouched by the schema addition.

Pre-existing lint failure unrelated to this change:
src/components/i18n/maintainership-banner.tsx triggers a
react-hooks/set-state-in-effect warning; the file is currently
untracked and lives outside this Wave's scope.

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

@@ -34,6 +34,7 @@ import {
 import { withIdempotency } from "@/lib/idempotency";
 import { mapAppleHealthEntry } from "@/lib/measurements/apple-health-mapping";
 import { validateMeasurementRange } from "@/lib/validations/measurement";
+import { deviceTypeEnum } from "@/lib/validations/source-priority";
 import { Prisma } from "@/generated/prisma/client";
 
 const MAX_BATCH_ENTRIES = 500;
@@ -47,6 +48,14 @@ const batchEntrySchema = z.object({
   sleepStage: z.number().int().min(0).max(20).optional(),
   externalId: z.string().min(1).max(120),
   externalSourceVersion: z.string().min(1).max(120).optional(),
+  // v1.4.25 W8c — optional device-type tag. The iOS client maps the
+  // `HKDevice.model` of each sample to one of the canonical device
+  // classes (watch | band | ring | phone | scale | other | unknown).
+  // Backward-compatible: every pre-W8c iOS build skips the field and
+  // the row is stored with `deviceType = null`; the canonical picker
+  // treats null as `unknown` and only uses it as a tiebreaker when a
+  // ranked device-type coexists in the same daily bucket.
+  deviceType: deviceTypeEnum.nullable().optional(),
 });
 
 const batchPayloadSchema = z.object({
@@ -156,6 +165,11 @@ async function postBatch(request: NextRequest): Promise<Response> {
         externalId: entry.externalId,
         externalSourceVersion: entry.externalSourceVersion ?? null,
         sleepStage: mapped.sleepStage ?? null,
+        // v1.4.25 W8c — pass the iOS-supplied device-type through to
+        // the row. Stays null for pre-W8c iOS builds; the canonical
+        // picker treats null as `unknown` so legacy ingest keeps
+        // working without a server-side default.
+        deviceType: entry.deviceType ?? null,
       },
     });
   }