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/models/ActiveWorkout.ts

@@ -0,0 +1,100 @@
+import mongoose from 'mongoose';
+
+export interface IActiveWorkoutSet {
+  reps: number | null;
+  weight: number | null;
+  rpe: number | null;
+  completed: boolean;
+}
+
+export interface IActiveWorkoutExercise {
+  exerciseId: string;
+  sets: IActiveWorkoutSet[];
+  restTime: number;
+}
+
+export interface IActiveWorkout {
+  _id?: string;
+  userId: string;
+  version: number;
+  name: string;
+  templateId: string | null;
+  exercises: IActiveWorkoutExercise[];
+  paused: boolean;
+  elapsed: number;
+  savedAt: number;
+  restStartedAt: number | null;
+  restTotal: number;
+  updatedAt?: Date;
+}
+
+const ActiveWorkoutSetSchema = new mongoose.Schema({
+  reps: { type: Number, default: null },
+  weight: { type: Number, default: null },
+  rpe: { type: Number, default: null },
+  completed: { type: Boolean, default: false }
+}, { _id: false });
+
+const ActiveWorkoutExerciseSchema = new mongoose.Schema({
+  exerciseId: { type: String, required: true, trim: true },
+  sets: { type: [ActiveWorkoutSetSchema], default: [] },
+  restTime: { type: Number, default: 120 }
+}, { _id: false });
+
+const ActiveWorkoutSchema = new mongoose.Schema(
+  {
+    userId: {
+      type: String,
+      required: true,
+      unique: true,
+      trim: true
+    },
+    version: {
+      type: Number,
+      required: true,
+      default: 1
+    },
+    name: {
+      type: String,
+      required: true,
+      trim: true,
+      maxlength: 100
+    },
+    templateId: {
+      type: String,
+      default: null
+    },
+    exercises: {
+      type: [ActiveWorkoutExerciseSchema],
+      default: []
+    },
+    paused: {
+      type: Boolean,
+      default: false
+    },
+    elapsed: {
+      type: Number,
+      default: 0
+    },
+    savedAt: {
+      type: Number,
+      default: () => Date.now()
+    },
+    restStartedAt: {
+      type: Number,
+      default: null
+    },
+    restTotal: {
+      type: Number,
+      default: 0
+    }
+  },
+  {
+    timestamps: true
+  }
+);
+
+// Auto-delete after 24h of inactivity
+ActiveWorkoutSchema.index({ updatedAt: 1 }, { expireAfterSeconds: 86400 });
+
+export const ActiveWorkout = mongoose.model<IActiveWorkout>('ActiveWorkout', ActiveWorkoutSchema);