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/models.py

@@ -1,5 +1,5 @@
 from datetime import date, datetime
-from sqlalchemy import Boolean, Integer, Date, DateTime, Text, Numeric, String, ForeignKey, UniqueConstraint, func
+from sqlalchemy import Boolean, Integer, Date, DateTime, Text, Numeric, String, ForeignKey, UniqueConstraint, Index, func
 from sqlalchemy.orm import Mapped, mapped_column
 from database import Base
 
@@ -30,6 +30,7 @@ class EggCollection(Base):
 
 class FlockHistory(Base):
     __tablename__ = "flock_history"
+    __table_args__ = (Index("ix_flock_history_user_date", "user_id", "date"),)
 
     id:            Mapped[int]      = mapped_column(Integer, primary_key=True, autoincrement=True)
     user_id:       Mapped[int]      = mapped_column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
@@ -41,6 +42,7 @@ class FlockHistory(Base):
 
 class FeedPurchase(Base):
     __tablename__ = "feed_purchases"
+    __table_args__ = (Index("ix_feed_purchases_user_date", "user_id", "date"),)
 
     id:            Mapped[int]      = mapped_column(Integer, primary_key=True, autoincrement=True)
     user_id:       Mapped[int]      = mapped_column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
@@ -53,6 +55,7 @@ class FeedPurchase(Base):
 
 class OtherPurchase(Base):
     __tablename__ = "other_purchases"
+    __table_args__ = (Index("ix_other_purchases_user_date", "user_id", "date"),)
 
     id:         Mapped[int]      = mapped_column(Integer, primary_key=True, autoincrement=True)
     user_id:    Mapped[int]      = mapped_column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)