fitness: add workout schedule rotation with next-workout suggestion

Users can define a custom order of templates (e.g., Push → Pull → Legs).
Based on the last completed session, the next workout in rotation is
recommended via a prominent banner and the floating action button.

- New WorkoutSchedule MongoDB model (per-user template order)
- GET/PUT /api/fitness/schedule API endpoints
- Schedule editor modal with reorder and add/remove
- Action button starts next scheduled workout when schedule exists

src/routes/api/fitness/schedule/+server.ts

@@ -0,0 +1,96 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { dbConnect } from '$utils/db';
+import { WorkoutSchedule } from '$models/WorkoutSchedule';
+import { WorkoutSession } from '$models/WorkoutSession';
+import { WorkoutTemplate } from '$models/WorkoutTemplate';
+import { requireAuth } from '$lib/server/middleware/auth';
+
+// GET /api/fitness/schedule - Get the user's workout schedule and next workout
+export const GET: RequestHandler = async ({ locals }) => {
+  const user = await requireAuth(locals);
+
+  try {
+    await dbConnect();
+
+    const schedule = await WorkoutSchedule.findOne({ userId: user.nickname });
+
+    if (!schedule || schedule.templateOrder.length === 0) {
+      return json({ schedule: null, nextTemplateId: null });
+    }
+
+    // Find the most recent session that used a template in the schedule
+    const lastSession = await WorkoutSession.findOne({
+      createdBy: user.nickname,
+      templateId: { $in: schedule.templateOrder }
+    }).sort({ startTime: -1 });
+
+    let nextTemplateId: string;
+
+    if (!lastSession?.templateId) {
+      // No previous session — start at the first template
+      nextTemplateId = schedule.templateOrder[0];
+    } else {
+      const lastId = lastSession.templateId.toString();
+      const idx = schedule.templateOrder.indexOf(lastId);
+      if (idx === -1) {
+        // Last session's template no longer in schedule — start at first
+        nextTemplateId = schedule.templateOrder[0];
+      } else {
+        // Next in rotation (wraps around)
+        nextTemplateId = schedule.templateOrder[(idx + 1) % schedule.templateOrder.length];
+      }
+    }
+
+    // Verify the template still exists
+    const templateExists = await WorkoutTemplate.exists({ _id: nextTemplateId });
+    if (!templateExists) {
+      nextTemplateId = schedule.templateOrder[0];
+    }
+
+    return json({
+      schedule: { templateOrder: schedule.templateOrder },
+      nextTemplateId
+    });
+  } catch (error) {
+    console.error('Error fetching workout schedule:', error);
+    return json({ error: 'Failed to fetch workout schedule' }, { status: 500 });
+  }
+};
+
+// PUT /api/fitness/schedule - Save the user's workout schedule (template order)
+export const PUT: RequestHandler = async ({ request, locals }) => {
+  const user = await requireAuth(locals);
+
+  try {
+    await dbConnect();
+
+    const { templateOrder } = await request.json();
+
+    if (!Array.isArray(templateOrder)) {
+      return json({ error: 'templateOrder must be an array' }, { status: 400 });
+    }
+
+    // Validate all template IDs belong to this user
+    if (templateOrder.length > 0) {
+      const count = await WorkoutTemplate.countDocuments({
+        _id: { $in: templateOrder },
+        createdBy: user.nickname
+      });
+      if (count !== templateOrder.length) {
+        return json({ error: 'Some template IDs are invalid' }, { status: 400 });
+      }
+    }
+
+    const schedule = await WorkoutSchedule.findOneAndUpdate(
+      { userId: user.nickname },
+      { templateOrder },
+      { upsert: true, new: true }
+    );
+
+    return json({ schedule: { templateOrder: schedule.templateOrder } });
+  } catch (error) {
+    console.error('Error saving workout schedule:', error);
+    return json({ error: 'Failed to save workout schedule' }, { status: 500 });
+  }
+};