Implement performance improvements across backend and frontend

- models.py: add composite (user_id, date) indexes to flock_history,
  feed_purchases, and other_purchases for faster date-filtered queries
  (egg_collections already had one via its unique constraint)
- main.py: add v2.2 migration to create the three composite indexes on
  existing installs at startup
- stats.py: fix N+1 query in monthly_stats — flock history is now fetched
  once and looked up per month using bisect_right instead of one DB query
  per month row; also remove unnecessary Decimal(str(...)) round-trips
  since SQLAlchemy already returns Numeric columns as Decimal
- eggs.py: add limit parameter (default 500, max 1000) to list_eggs to
  cap unbounded fetches on large datasets
- dashboard.js: pass start= (30 days ago) when fetching eggs so the
  dashboard only loads the data it actually needs for the chart and
  recent collections list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx/html/js/dashboard.js

@@ -80,11 +80,16 @@ async function loadDashboard() {
     const msg = document.getElementById('msg');
 
     try {
-        // Fetch stats and recent eggs in parallel
+        // Fetch stats and recent eggs in parallel; eggs limited to last 30 days for chart + recent list
+        const tz = (typeof Auth !== 'undefined' && Auth.getUser()?.timezone) || 'UTC';
+        const start30 = new Date();
+        start30.setDate(start30.getDate() - 30);
+        const start30str = start30.toLocaleDateString('en-CA', { timeZone: tz });
+
         const [stats, budget, eggs] = await Promise.all([
             API.get('/api/stats/dashboard'),
             API.get('/api/stats/budget'),
-            API.get('/api/eggs'),
+            API.get(`/api/eggs?start=${start30str}`),
         ]);
 
         // Populate stat cards