feat(api): DELETE /api/measurements/by-external-ids for iOS deletion-sync

HealthKit's API does not surface a stream of deleted UUIDs, so the iOS
HealthLog client performs a periodic 30-day reconciliation: it diffs
HealthKit's current UUID set against the server's externalIds and
posts the surplus here. This endpoint deletes the user's matching
rows and never touches another user's data (Prisma's deleteMany with
the userId predicate is the cross-user 404 guard).

Contract:
  - Body: { externalIds: string[] } with min 0, max 500 entries.
  - Auth: requireAuth() — session cookie or bearer.
  - Empty array: returns { deletedCount: 0 } with 200 (no-op, not a
    failure — matches the ingest path's symmetric tolerance).
  - Over-cap: 422 with errorCode "measurement.delete.too_large" so
    the iOS client can surface a specific diagnostic.

Integration coverage: happy path, cross-user 404 guard, empty array,
and batch-cap rejection. The cap (500) mirrors POST /measurements/batch
so the two surfaces share one operational ceiling.

OpenAPI: registered as DELETE on /api/measurements/by-external-ids
with the Measurements tag; req/res schemas reuse the same z.array cap
the runtime validator enforces. openapi.yaml regenerated.

src/lib/openapi/routes.ts

@@ -267,6 +267,22 @@ const batchResponse = z
   })
   .meta({ id: "AppleHealthBatchResponse" });
 
+const deleteByExternalIdsRequest = z
+  .object({
+    externalIds: z.array(z.string().min(1).max(120)).min(0).max(500),
+  })
+  .meta({
+    id: "MeasurementsDeleteByExternalIdsRequest",
+    description:
+      "iOS deletion-sync. Up to 500 externalIds per call; matching rows owned by other users are silently skipped (cross-user 404 guard).",
+  });
+
+const deleteByExternalIdsResponse = z
+  .object({
+    deletedCount: z.number().int().nonnegative(),
+  })
+  .meta({ id: "MeasurementsDeleteByExternalIdsResult" });
+
 const measurementResource = z
   .object({
     id: z.string(),
@@ -467,6 +483,34 @@ export const openApiPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       },
     },
   },
+  "/api/measurements/by-external-ids": {
+    delete: {
+      tags: ["Measurements"],
+      summary: "Delete measurements by external ID (iOS deletion-sync)",
+      description:
+        "Removes the user's measurement rows whose externalId is in the request list. Rows owned by another user are silently skipped (cross-user 404 guard). Up to 500 externalIds per call.",
+      requestBody: {
+        required: true,
+        content: {
+          "application/json": { schema: deleteByExternalIdsRequest },
+        },
+      },
+      responses: {
+        "200": {
+          description: "Delete batch processed.",
+          content: {
+            "application/json": {
+              schema: dataEnvelope(
+                deleteByExternalIdsResponse,
+                "MeasurementsDeleteByExternalIdsResponse",
+              ),
+            },
+          },
+        },
+        ...stdResponses,
+      },
+    },
+  },
   "/api/devices": {
     post: {
       tags: ["Devices"],