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/models/WorkoutSchedule.ts

@@ -0,0 +1,31 @@
+import mongoose from 'mongoose';
+
+export interface IWorkoutSchedule {
+  _id?: string;
+  userId: string;
+  templateOrder: string[]; // array of WorkoutTemplate _id strings in rotation order
+  createdAt?: Date;
+  updatedAt?: Date;
+}
+
+const WorkoutScheduleSchema = new mongoose.Schema(
+  {
+    userId: {
+      type: String,
+      required: true,
+      unique: true,
+      trim: true
+    },
+    templateOrder: {
+      type: [String],
+      default: []
+    }
+  },
+  {
+    timestamps: true,
+    toJSON: { virtuals: true },
+    toObject: { virtuals: true }
+  }
+);
+
+export const WorkoutSchedule = mongoose.model<IWorkoutSchedule>('WorkoutSchedule', WorkoutScheduleSchema);