feat(subscriptions): migrate subscription management to merchant level and seed tiers

Move subscription create/edit from store detail (broken endpoint) to merchant
detail page with proper modal UI. Seed 4 subscription tiers (Essential,
Professional, Business, Enterprise) in init_production.py. Also includes
cross-module dependency declarations, store domain platform_id migration,
platform context middleware, CMS route fixes, and migration backups.

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

alembic/versions/z_add_platform_id_to_store_domains.py

@@ -0,0 +1,48 @@
+"""add platform_id to store_domains
+
+Revision ID: z_store_domain_platform_id
+Revises: core_001
+Create Date: 2026-02-08
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "z_store_domain_platform_id"
+down_revision = "core_001"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Add platform_id column
+    op.add_column(
+        "store_domains",
+        sa.Column(
+            "platform_id",
+            sa.Integer(),
+            sa.ForeignKey("platforms.id", ondelete="SET NULL"),
+            nullable=True,
+            comment="Platform this domain is associated with (for platform context resolution)",
+        ),
+    )
+    op.create_index("idx_store_domain_platform", "store_domains", ["platform_id"])
+
+    # Backfill: set platform_id from the store's primary store_platform
+    op.execute(
+        """
+        UPDATE store_domains sd
+        SET platform_id = (
+            SELECT sp.platform_id
+            FROM store_platforms sp
+            WHERE sp.store_id = sd.store_id
+              AND sp.is_primary = true
+            LIMIT 1
+        )
+        WHERE sd.platform_id IS NULL
+        """
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("idx_store_domain_platform", table_name="store_domains")
+    op.drop_column("store_domains", "platform_id")