Fix order saving, isMyOrder, blocked waiters, options pricing; add preferences, table groups, product images

Backend:
- OrderItemInput accepts option objects {id,name,price_delta} instead of int IDs
- extra_cost from selected options added to unit_price snapshot
- GET /api/products/?all=true for manager (includes unavailable)
- PUT /api/products/{id} now replaces options, ingredients, preference_sets
- POST /api/products/{id}/image — persistent image upload to /app/data/product_images
- New models: ProductPreferenceSet, ProductPreferenceChoice, TableGroup
- tables: group_id FK, hard delete (?hard=true), batch create POST /api/tables/batch
- GET /api/tables/groups + POST/PUT/DELETE groups endpoints
- POST /api/auth/me endpoint for token rehydration
- Auto-migration on startup for new columns

PWA:
- AuthRehydrator: fetches /auth/me on load so isMyOrder works after page reload
- 401 response force-logs out (covers blocked waiters)
- ItemOptionsModal: uses extra_cost correctly, shows preferences as radio buttons

Manager:
- ProductsPage: shows unavailable products greyed out, category color picker + reorder,
  full option/ingredient/preference editing, image upload
- TablesPage: table groups, auto-increment, deactivate vs hard delete, batch add

local_backend/models/table.py

@@ -1,16 +1,28 @@
-from sqlalchemy import Column, Integer, String, Boolean, Float
+from sqlalchemy import Column, Integer, String, Boolean, Float, ForeignKey
 from sqlalchemy.orm import relationship
 from database import Base
 
 
+class TableGroup(Base):
+    __tablename__ = "table_groups"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String, nullable=False, unique=True)
+    sort_order = Column(Integer, default=0)
+
+    tables = relationship("Table", back_populates="group")
+
+
 class Table(Base):
     __tablename__ = "tables"
 
     id = Column(Integer, primary_key=True, index=True)
-    number = Column(Integer, unique=True, nullable=False)
+    number = Column(Integer, nullable=False)
     label = Column(String, nullable=True)
+    group_id = Column(Integer, ForeignKey("table_groups.id"), nullable=True)
     is_active = Column(Boolean, default=True, nullable=False)
     floor_x = Column(Float, nullable=True)
     floor_y = Column(Float, nullable=True)
 
+    group = relationship("TableGroup", back_populates="tables")
     orders = relationship("Order", back_populates="table")