fitness: add 5 default workout templates and new exercises

Add all 5 PPL+Upper/Lower templates matching the target split,
with Day 3 (Legs) adjusted to include Bulgarian split squats and
standing calf raises, and Day 5 (Lower) reworked with front squats,
hip thrusts, and goblet squats — all equipment-free of machines.

Also adds incline row, decline crunch, flat leg raise, and nordic
hamstring curl to the exercise list, and updates the WorkoutTemplate
model to use exerciseId instead of name for exercise references.

src/models/WorkoutTemplate.ts

@@ -7,7 +7,8 @@ export interface ISet {
 }
 
 export interface IExercise {
-  name: string;
+  exerciseId: string;
+  name?: string;
   sets: ISet[];
   restTime?: number; // Rest time in seconds, defaults to 120 (2 minutes)
 }
@@ -43,11 +44,15 @@ const SetSchema = new mongoose.Schema({
 });
 
 const ExerciseSchema = new mongoose.Schema({
-  name: {
+  exerciseId: {
     type: String,
     required: true,
     trim: true
   },
+  name: {
+    type: String,
+    trim: true
+  },
   sets: {
     type: [SetSchema],
     required: true,