fitness: add multi-device workout sync via SSE and rest timer improvements

Enables real-time workout synchronization across devices using
Server-Sent Events and an ephemeral MongoDB document (24h TTL).
Rest timers now use absolute timestamps instead of interval-based
countdown for accurate cross-device sync. Adds +/-30s rest timer
adjust buttons.

src/routes/api/fitness/workout/active/+server.ts

@@ -0,0 +1,105 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { dbConnect } from '$utils/db';
+import { ActiveWorkout } from '$models/ActiveWorkout';
+import { broadcast } from '$lib/server/sseManager';
+
+// GET /api/fitness/workout/active — fetch current active workout
+export const GET: RequestHandler = async ({ locals }) => {
+  const session = await locals.auth();
+  if (!session?.user?.nickname) {
+    return json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  try {
+    await dbConnect();
+    const doc = await ActiveWorkout.findOne({ userId: session.user.nickname }).lean();
+    if (!doc) {
+      return json({ active: false });
+    }
+    return json({ active: true, workout: doc });
+  } catch (error) {
+    console.error('Error fetching active workout:', error);
+    return json({ error: 'Failed to fetch active workout' }, { status: 500 });
+  }
+};
+
+// PUT /api/fitness/workout/active — create or update active workout state
+export const PUT: RequestHandler = async ({ request, locals }) => {
+  const session = await locals.auth();
+  if (!session?.user?.nickname) {
+    return json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  try {
+    await dbConnect();
+    const data = await request.json();
+    const { name, templateId, exercises, paused, elapsed, savedAt, expectedVersion, restStartedAt, restTotal } = data;
+
+    if (!name) {
+      return json({ error: 'Name is required' }, { status: 400 });
+    }
+
+    const userId = session.user.nickname;
+    const existing = await ActiveWorkout.findOne({ userId });
+
+    if (existing && expectedVersion != null && existing.version !== expectedVersion) {
+      // Conflict — client is out of date
+      return json(
+        { error: 'Version conflict', workout: existing },
+        { status: 409 }
+      );
+    }
+
+    const newVersion = existing ? existing.version + 1 : 1;
+
+    const doc = await ActiveWorkout.findOneAndUpdate(
+      { userId },
+      {
+        $set: {
+          name,
+          templateId: templateId ?? null,
+          exercises: exercises ?? [],
+          paused: paused ?? false,
+          elapsed: elapsed ?? 0,
+          savedAt: savedAt ?? Date.now(),
+          restStartedAt: restStartedAt ?? null,
+          restTotal: restTotal ?? 0,
+          version: newVersion
+        },
+        $setOnInsert: { userId }
+      },
+      { upsert: true, new: true, lean: true }
+    );
+
+    // Broadcast to all other connected devices
+    broadcast(userId, 'update', doc);
+
+    return json({ workout: doc });
+  } catch (error) {
+    console.error('Error updating active workout:', error);
+    return json({ error: 'Failed to update active workout' }, { status: 500 });
+  }
+};
+
+// DELETE /api/fitness/workout/active — clear active workout (finish/cancel)
+export const DELETE: RequestHandler = async ({ locals }) => {
+  const session = await locals.auth();
+  if (!session?.user?.nickname) {
+    return json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  try {
+    await dbConnect();
+    const userId = session.user.nickname;
+    await ActiveWorkout.deleteOne({ userId });
+
+    // Notify all devices that workout is finished
+    broadcast(userId, 'finished', { active: false });
+
+    return json({ ok: true });
+  } catch (error) {
+    console.error('Error deleting active workout:', error);
+    return json({ error: 'Failed to delete active workout' }, { status: 500 });
+  }
+};