Add Other Purchases to budget page

- New other_purchases table (date, total, notes)
- /api/other CRUD endpoints
- Budget stats now include other costs in cost/egg and cost/dozen math
- Budget page: new Log Other Purchases form, stat cards for other costs,
  combined Purchase History table showing feed and other entries together

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

backend/models.py

@@ -33,3 +33,13 @@ class FeedPurchase(Base):
     price_per_bag: Mapped[float]    = mapped_column(Numeric(10, 2), nullable=False)
     notes:         Mapped[str]      = mapped_column(Text, nullable=True)
     created_at:    Mapped[datetime] = mapped_column(DateTime, server_default=func.now())
+
+
+class OtherPurchase(Base):
+    __tablename__ = "other_purchases"
+
+    id:         Mapped[int]      = mapped_column(Integer, primary_key=True, autoincrement=True)
+    date:       Mapped[date]     = mapped_column(Date, nullable=False, index=True)
+    total:      Mapped[float]    = mapped_column(Numeric(10, 2), nullable=False)
+    notes:      Mapped[str]      = mapped_column(Text, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())