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/eggs.py

@@ -17,13 +17,16 @@ router = APIRouter(prefix="/api/eggs", tags=["eggs"])
 def list_eggs(
     start: Optional[date] = None,
     end:   Optional[date] = None,
+    limit: int = 500,
     db:    Session = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
+    limit = min(limit, 1000)
     q = (
         select(EggCollection)
         .where(EggCollection.user_id == current_user.id)
         .order_by(EggCollection.date.desc())
+        .limit(limit)
     )
     if start:
         q = q.where(EggCollection.date >= start)