Take the writes off three read routes

A GET must not have side effects — api-handler.ts says so next to
READ_HTTP_METHODS, where an MCP-audience token is admitted on the safe
methods precisely because they are assumed to write nothing. Three routes
broke that: the sync handshake bumped User.lastSyncedAt, the achievements
read inserted unlock rows, and the cycle calendar fired a discarded promise
at the forecast cache so a failed write left no trace anywhere.

Each write moves to the place that causes it.

The sync checkpoint is now stamped by the batch ingest endpoints for
measurements, mood and intakes. The handshake reports it. That also fixes
the watermark: the checkpoint used to advance past a window the client had
not drained, and a client trusting it would skip that window. It can only
trail a real sync now.

The unlock rows move to a nightly achievement-unlock-sweep. The badge grid
never needed them — unlocked comes from the live metrics and the completion
date is derived — so the payload is unchanged; the rows pin that date and
carry it into the backup, and the sweep reaches accounts that never open
the page.

The cycle forecast moves to a nightly cycle-prediction-refresh. The
CyclePrediction row is not a cache of a read: it is the index the reminder
cron scans, so which reminders an account received depended on whether
somebody had opened the calendar. persistPredictionCache no longer
swallows its errors either — the job reports them.

src/app/api/sync/state/route.ts

@@ -12,7 +12,9 @@
  *   {
  *     userId,
  *     timezone,
- *     lastSyncedAt,             — User.lastSyncedAt (ISO string or null)
+ *     lastSyncedAt,             — User.lastSyncedAt (ISO string or null).
+ *                                 Set by the ingest boundaries, not here:
+ *                                 this route is a read and writes nothing.
  *     serverNow,                — `new Date().toISOString()` for clock skew
  *     measurements: { lastUpdatedAt, liveCount, tombstonedCount },
  *     mood:         { lastUpdatedAt, liveCount, tombstonedCount },
@@ -45,10 +47,10 @@ import {
 export const GET = apiHandler(async (_request: NextRequest) => {
   const { user } = await requireAuth();
 
-  // Per-domain aggregate reads + one user-side bump. Each runs against
-  // indexed columns (userId, deleted_at / updated_at) so it stays
-  // O(rows-for-user) and not O(rows-across-tenants). v1.7.0 adds the
-  // mood + intake blocks now that both carry the sync columns.
+  // Per-domain aggregate reads. Each runs against indexed columns (userId,
+  // deleted_at / updated_at) so it stays O(rows-for-user) and not
+  // O(rows-across-tenants). v1.7.0 adds the mood + intake blocks now that
+  // both carry the sync columns.
   const [
     measurementLatest,
     measurementLive,
@@ -96,16 +98,14 @@ export const GET = apiHandler(async (_request: NextRequest) => {
     }),
   ]);
 
-  // The handshake also bumps the user's lastSyncedAt — the call IS
-  // the handshake. iOS reads the OLD lastSyncedAt from the response
-  // and then trusts that subsequent server writes after the new
-  // checkpoint will round-trip via the standard read paths.
-  const previous = currentUser?.lastSyncedAt ?? null;
-  const nextCheckpoint = new Date();
-  await prisma.user.update({
-    where: { id: user.id },
-    data: { lastSyncedAt: nextCheckpoint },
-  });
+  // The handshake reports the checkpoint; it does not set it. The column is
+  // stamped by the ingest boundaries that actually move data (see
+  // `markSyncCheckpoint`), so `lastSyncedAt` answers "when did this account
+  // last push a batch" rather than "when did a client last ask". NULL still
+  // means never synced, and the value can now only trail the truth — the
+  // handshake used to advance it past a window the client had not drained.
+  const lastSyncedAt = currentUser?.lastSyncedAt ?? null;
+  const serverNow = new Date();
 
   annotate({
     action: { name: "sync.state" },
@@ -114,15 +114,15 @@ export const GET = apiHandler(async (_request: NextRequest) => {
       tombstonedCount: measurementTomb,
       moodLiveCount: moodLive,
       intakeLiveCount: intakeLive,
-      lastSyncedAtBefore: previous?.toISOString() ?? null,
+      lastSyncedAt: lastSyncedAt?.toISOString() ?? null,
     },
   });
 
   return apiSuccess({
     userId: user.id,
     timezone: currentUser?.timezone ?? "Europe/Berlin",
-    lastSyncedAt: previous?.toISOString() ?? null,
-    serverNow: nextCheckpoint.toISOString(),
+    lastSyncedAt: lastSyncedAt?.toISOString() ?? null,
+    serverNow: serverNow.toISOString(),
     measurements: {
       lastUpdatedAt: measurementLatest?.updatedAt?.toISOString() ?? null,
       liveCount: measurementLive,