feat(connect): Phase 2 — cloud DB models for Xenia Connect

Adds three new tables created automatically by create_all on next
startup, plus an additive migration for an existing table:

models/menu_snapshot.py (NEW)
  - menu_snapshots: one row per site, holds the full serialized
    category+product JSON; upserted on each sync from local_backend

models/online_order.py (NEW)
  - online_orders: full order lifecycle from customer submission
    through delivery; tracks sync state so local_backend can poll
    for unprocessed orders (synced_to_local=0)

models/manager_account.py (NEW)
  - manager_accounts + manager_site_access (M2M join table): remote
    manager login accounts with per-site access control

models/site.py (MODIFIED)
  - order_counter column added; incremented atomically when each
    online order is created to generate "ORD-XXXX" public_ref values

main.py (MODIFIED)
  - imports all three new models so create_all registers their tables
  - _run_migrations() added (same pattern as local_backend) for the
    additive order_counter column on the existing sites table

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

cloud_backend/models/menu_snapshot.py

@@ -0,0 +1,12 @@
+from sqlalchemy import Column, Integer, Text, DateTime, ForeignKey
+from sqlalchemy.sql import func
+from database import Base
+
+
+class MenuSnapshot(Base):
+    __tablename__ = "menu_snapshots"
+
+    id = Column(Integer, primary_key=True, index=True)
+    site_id = Column(Integer, ForeignKey("sites.id"), nullable=False, index=True, unique=True)
+    snapshot_json = Column(Text, nullable=False)
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())