perf: add projection + O(1) bucket math to muscle-heatmap endpoint

Endpoint previously pulled full WorkoutSession documents (including
gpsTrack, notes, kcalEstimate etc.) to count sets per muscle group.
Adds a projection that keeps only startTime + exercises.exerciseId +
whole set objects — safe (avoids the malformed-sub-array issue the
earlier narrower projection caused in the stats overview handler),
but still drops the bulky session-level fields.

Also swaps the per-session findIndex() over the weekly bucket array
for direct date-math against the first bucket's Monday, turning
bucket lookup from O(sessions × weeks) into O(sessions).

TODO.md

@@ -10,7 +10,7 @@ Order = impact. Font items + app.html preload intentionally skipped.
 - [x] 4. Favorites page — drop unnecessary `all_brief` fetch (verified Search uses `favoritesOnly` so `allRecipes` was redundant)
 - [x] 5. Replace redundant `locals.auth()` with `locals.session` across all routes (68 files, 107 sites — loaders, actions, API endpoints)
 - [x] 6. Stream fitness stats loader — muscleHeatmap, nutritionStats, periods, sharedPeriods now stream via `{#await}`. `stats` still awaited (too many chart $deriveds depend on it)
-- [ ] 7. Overview endpoint — add `.select(...)` projection, cap timeseries window
+- [x] 7. Muscle-heatmap endpoint — add projection + O(1) bucket math. Overview already had a projection; set-subfield narrowing was attempted but reverted (returned malformed sets). Timeseries cap not feasible: totals are lifetime-scoped.
 - [ ] 8. Calendar payload trim — drop `name` from `yearDays`, pre-filter `feastDots` server-side
 - [ ] 9. History sessions endpoint — slim exercise payload for list view
 - [ ] 10. `Cache-Control` headers on stable API endpoints (all_brief, calendar, exercises metadata)

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.46.20",
+	"version": "1.46.21",
 	"private": true,
 	"type": "module",
 	"scripts": {

src/routes/api/fitness/stats/muscle-heatmap/+server.ts

@@ -21,20 +21,19 @@ export const GET: RequestHandler = async ({ url, locals }) => {
 	const since = new Date();
 	since.setDate(since.getDate() - weeks * 7);
 
-	const sessions = await WorkoutSession.find({
-		createdBy: user.nickname,
-		startTime: { $gte: since }
-	}).lean();
+	const sessions = await WorkoutSession.find(
+		{ createdBy: user.nickname, startTime: { $gte: since } },
+		{ startTime: 1, 'exercises.exerciseId': 1, 'exercises.sets': 1 }
+	).lean();
 
 	// Build weekly buckets
 	type MuscleData = { primary: number; secondary: number };
 	const weeklyData: { weekStart: string; muscles: Record<string, MuscleData> }[] = [];
 
-	// Initialize week buckets
+	// Initialize week buckets (Monday-aligned)
 	for (let w = 0; w < weeks; w++) {
 		const d = new Date();
 		d.setDate(d.getDate() - (weeks - 1 - w) * 7);
-		// Find Monday of that week
 		const day = d.getDay();
 		const diff = d.getDate() - day + (day === 0 ? -6 : 1);
 		d.setDate(diff);
@@ -47,18 +46,16 @@ export const GET: RequestHandler = async ({ url, locals }) => {
 		weeklyData.push({ weekStart, muscles });
 	}
 
+	// Map a session's timestamp straight to a bucket index using the first
+	// bucket's Monday as the epoch, avoiding a linear findIndex per session.
+	const firstWeekStartMs = weeklyData.length > 0 ? new Date(weeklyData[0].weekStart).getTime() : 0;
+	const weekMs = 7 * 86400000;
+
 	// Aggregate muscle usage
 	for (const session of sessions) {
 		const sessionDate = new Date(session.startTime);
-		// Find which week bucket
-		const weekIdx = weeklyData.findIndex((w, i) => {
-			const start = new Date(w.weekStart);
-			const nextStart = i + 1 < weeklyData.length
-				? new Date(weeklyData[i + 1].weekStart)
-				: new Date(start.getTime() + 7 * 86400000);
-			return sessionDate >= start && sessionDate < nextStart;
-		});
-		if (weekIdx === -1) continue;
+		const weekIdx = Math.floor((sessionDate.getTime() - firstWeekStartMs) / weekMs);
+		if (weekIdx < 0 || weekIdx >= weeklyData.length) continue;
 
 		const bucket = weeklyData[weekIdx].muscles;