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/routes/api/fitness/goal/+server.ts

@@ -0,0 +1,111 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { requireAuth } from '$lib/server/middleware/auth';
+import { dbConnect } from '$utils/db';
+import { FitnessGoal } from '$models/FitnessGoal';
+import { WorkoutSession } from '$models/WorkoutSession';
+
+export const GET: RequestHandler = async ({ locals }) => {
+	const user = await requireAuth(locals);
+	await dbConnect();
+
+	const goal = await FitnessGoal.findOne({ username: user.nickname }).lean() as any;
+	const weeklyWorkouts = goal?.weeklyWorkouts ?? null;
+
+	// If no goal set, return early
+	if (weeklyWorkouts === null) {
+		return json({ weeklyWorkouts: null, streak: 0 });
+	}
+
+	const streak = await computeStreak(user.nickname, weeklyWorkouts);
+	return json({ weeklyWorkouts, streak });
+};
+
+export const PUT: RequestHandler = async ({ request, locals }) => {
+	const user = await requireAuth(locals);
+	const { weeklyWorkouts } = await request.json();
+
+	if (typeof weeklyWorkouts !== 'number' || weeklyWorkouts < 1 || weeklyWorkouts > 14 || !Number.isInteger(weeklyWorkouts)) {
+		return json({ error: 'weeklyWorkouts must be an integer between 1 and 14' }, { status: 400 });
+	}
+
+	await dbConnect();
+
+	await FitnessGoal.findOneAndUpdate(
+		{ username: user.nickname },
+		{ weeklyWorkouts },
+		{ upsert: true }
+	);
+
+	const streak = await computeStreak(user.nickname, weeklyWorkouts);
+	return json({ weeklyWorkouts, streak });
+};
+
+async function computeStreak(username: string, weeklyGoal: number): Promise<number> {
+	// Get weekly workout counts going back up to 2 years
+	const cutoff = new Date();
+	cutoff.setFullYear(cutoff.getFullYear() - 2);
+
+	const weeklyAgg = await WorkoutSession.aggregate([
+		{
+			$match: {
+				createdBy: username,
+				startTime: { $gte: cutoff }
+			}
+		},
+		{
+			$group: {
+				_id: {
+					year: { $isoWeekYear: '$startTime' },
+					week: { $isoWeek: '$startTime' }
+				},
+				count: { $sum: 1 }
+			}
+		},
+		{
+			$sort: { '_id.year': -1, '_id.week': -1 }
+		}
+	]);
+
+	// Build a set of weeks that met the goal
+	const metGoal = new Set<string>();
+	for (const item of weeklyAgg) {
+		if (item.count >= weeklyGoal) {
+			metGoal.add(`${item._id.year}-${item._id.week}`);
+		}
+	}
+
+	// Walk backwards week-by-week counting consecutive weeks that met the goal.
+	// Current (incomplete) week counts if it already meets the goal, otherwise skip it.
+	const now = new Date();
+	let streak = 0;
+
+	const currentKey = isoWeekKey(now);
+	const currentWeekMet = metGoal.has(currentKey);
+
+	// If current week already met: count it, then check previous weeks.
+	// If not: start checking from last week (current week still in progress).
+	if (currentWeekMet) streak = 1;
+
+	for (let i = 1; i <= 104; i++) {
+		const weekDate = new Date(now);
+		weekDate.setDate(weekDate.getDate() - i * 7);
+		if (metGoal.has(isoWeekKey(weekDate))) {
+			streak++;
+		} else {
+			break;
+		}
+	}
+
+	return streak;
+}
+
+function isoWeekKey(date: Date): string {
+	const d = new Date(date.getTime());
+	d.setHours(0, 0, 0, 0);
+	d.setDate(d.getDate() + 3 - ((d.getDay() + 6) % 7));
+	const year = d.getFullYear();
+	const week1 = new Date(year, 0, 4);
+	const week = 1 + Math.round(((d.getTime() - week1.getTime()) / 86400000 - 3 + ((week1.getDay() + 6) % 7)) / 7);
+	return `${year}-${week}`;
+}