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>

backend/routers/stats.py

@@ -1,4 +1,5 @@
 import calendar
+from bisect import bisect_right
 from datetime import date, datetime, timedelta
 from decimal import Decimal
 from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
@@ -170,20 +171,24 @@ def monthly_stats(
 
     other_map = {(r.year, r.month): r.other_cost for r in other_rows}
 
+    # Fetch all flock history once (ascending) to avoid N+1 per month
+    flock_all = db.scalars(
+        select(FlockHistory)
+        .where(FlockHistory.user_id == uid)
+        .order_by(FlockHistory.date)
+    ).all()
+    flock_dates  = [f.date for f in flock_all]
+    flock_counts = [f.chicken_count for f in flock_all]
+
+    def flock_at(month_end: date) -> int | None:
+        idx = bisect_right(flock_dates, month_end) - 1
+        return flock_counts[idx] if idx >= 0 else None
+
     results = []
     for row in egg_rows:
         y, m = int(row.year), int(row.month)
-        last_day  = calendar.monthrange(y, m)[1]
-        month_end = date(y, m, last_day)
-
-        flock_row = db.scalars(
-            select(FlockHistory)
-            .where(FlockHistory.user_id == uid)
-            .where(FlockHistory.date <= month_end)
-            .order_by(FlockHistory.date.desc())
-            .limit(1)
-        ).first()
-        flock = flock_row.chicken_count if flock_row else None
+        month_end = date(y, m, calendar.monthrange(y, m)[1])
+        flock = flock_at(month_end)
 
         total_eggs  = int(row.total_eggs)
         days_logged = int(row.days_logged)
@@ -192,11 +197,11 @@ def monthly_stats(
 
         raw_feed_cost  = feed_map.get((y, m))
         raw_other_cost = other_map.get((y, m))
-        feed_cost  = round(Decimal(str(raw_feed_cost)),  2) if raw_feed_cost  else None
-        other_cost = round(Decimal(str(raw_other_cost)), 2) if raw_other_cost else None
+        feed_cost  = round(raw_feed_cost,  2) if raw_feed_cost  else None
+        other_cost = round(raw_other_cost, 2) if raw_other_cost else None
 
-        raw_total_cost = (raw_feed_cost or 0) + (raw_other_cost or 0)
-        cpe = round(Decimal(str(raw_total_cost)) / Decimal(total_eggs), 4) if (raw_total_cost and total_eggs) else None
+        total_cost = (raw_feed_cost or Decimal(0)) + (raw_other_cost or Decimal(0))
+        cpe = round(total_cost / total_eggs, 4) if (total_cost and total_eggs) else None
         cpd = round(cpe * 12, 4) if cpe else None
 
         results.append(MonthlySummary(