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>

alembic/versions/k9f0a1b2c3d4_add_tier_id_fk_to_subscriptions.py

@@ -0,0 +1,71 @@
+"""Add tier_id FK to vendor_subscriptions
+
+Revision ID: k9f0a1b2c3d4
+Revises: 2953ed10d22c
+Create Date: 2025-12-26
+
+Adds tier_id column to vendor_subscriptions table with FK to subscription_tiers.
+Backfills tier_id based on existing tier (code) values.
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "k9f0a1b2c3d4"
+down_revision = "2953ed10d22c"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Use batch mode for SQLite compatibility
+    with op.batch_alter_table("vendor_subscriptions", schema=None) as batch_op:
+        # Add tier_id column (nullable for backfill)
+        batch_op.add_column(
+            sa.Column("tier_id", sa.Integer(), nullable=True)
+        )
+        # Create index for tier_id
+        batch_op.create_index(
+            "ix_vendor_subscriptions_tier_id",
+            ["tier_id"],
+            unique=False,
+        )
+        # Add FK constraint
+        batch_op.create_foreign_key(
+            "fk_vendor_subscriptions_tier_id",
+            "subscription_tiers",
+            ["tier_id"],
+            ["id"],
+            ondelete="SET NULL",
+        )
+
+    # Backfill tier_id from tier code
+    # This updates existing subscriptions to link to their tier
+    op.execute(
+        """
+        UPDATE vendor_subscriptions
+        SET tier_id = (
+            SELECT id FROM subscription_tiers
+            WHERE subscription_tiers.code = vendor_subscriptions.tier
+        )
+        WHERE EXISTS (
+            SELECT 1 FROM subscription_tiers
+            WHERE subscription_tiers.code = vendor_subscriptions.tier
+        )
+        """
+    )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("vendor_subscriptions", schema=None) as batch_op:
+        # Drop FK constraint
+        batch_op.drop_constraint(
+            "fk_vendor_subscriptions_tier_id",
+            type_="foreignkey",
+        )
+        # Drop index
+        batch_op.drop_index("ix_vendor_subscriptions_tier_id")
+        # Drop column
+        batch_op.drop_column("tier_id")