feat: add tier_id FK to VendorSubscription for proper tier relationship

- Add tier_id column with FK to subscription_tiers table
- Add tier_obj relationship to VendorSubscription model
- Update tier_limits property to use database tier when available
- Create migration with SQLite batch mode support
- Backfill tier_id from existing tier code values

This enables proper database relationship between vendors and their
subscription tier, instead of just storing the tier code string.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/subscription.py

@@ -431,7 +431,10 @@ class VendorSubscription(Base, TimestampMixin):
         Integer, ForeignKey("vendors.id"), unique=True, nullable=False, index=True
     )
 
-    # Tier
+    # Tier - tier_id is the FK, tier (code) kept for backwards compatibility
+    tier_id = Column(
+        Integer, ForeignKey("subscription_tiers.id"), nullable=True, index=True
+    )
     tier = Column(
         String(20), default=TierCode.ESSENTIAL.value, nullable=False, index=True
     )
@@ -479,6 +482,7 @@ class VendorSubscription(Base, TimestampMixin):
 
     # Relationships
     vendor = relationship("Vendor", back_populates="subscription")
+    tier_obj = relationship("SubscriptionTier", backref="subscriptions")
 
     __table_args__ = (
         Index("idx_subscription_vendor_status", "vendor_id", "status"),
@@ -494,7 +498,20 @@ class VendorSubscription(Base, TimestampMixin):
 
     @property
     def tier_limits(self) -> dict:
-        """Get the limit definitions for current tier."""
+        """Get the limit definitions for current tier.
+
+        Uses database tier (tier_obj) if available, otherwise falls back
+        to hardcoded TIER_LIMITS for backwards compatibility.
+        """
+        # Use database tier if relationship is loaded
+        if self.tier_obj is not None:
+            return {
+                "orders_per_month": self.tier_obj.orders_per_month,
+                "products_limit": self.tier_obj.products_limit,
+                "team_members": self.tier_obj.team_members,
+                "features": self.tier_obj.features or [],
+            }
+        # Fall back to hardcoded limits
         return TIER_LIMITS.get(TierCode(self.tier), TIER_LIMITS[TierCode.ESSENTIAL])
 
     @property