fitness: add weekly workout goal with streak counter on stats page

Store a per-user weekly workout target (1-14) in a new FitnessGoal model.
Compute consecutive-week streak from WorkoutSession history via a new
/api/fitness/goal endpoint. Display streak as a 4th lifetime card on the
stats page with an inline goal editor modal.

src/models/FitnessGoal.ts

@@ -0,0 +1,18 @@
+import mongoose from 'mongoose';
+
+const FitnessGoalSchema = new mongoose.Schema(
+  {
+    username: { type: String, required: true, unique: true },
+    weeklyWorkouts: { type: Number, required: true, default: 4, min: 1, max: 14 }
+  },
+  { timestamps: true }
+);
+
+interface IFitnessGoal {
+  username: string;
+  weeklyWorkouts: number;
+}
+
+let _model: mongoose.Model<IFitnessGoal>;
+try { _model = mongoose.model<IFitnessGoal>("FitnessGoal"); } catch { _model = mongoose.model<IFitnessGoal>("FitnessGoal", FitnessGoalSchema); }
+export const FitnessGoal = _model;