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>

app/modules/tenancy/models/store_domain.py

@@ -52,14 +52,25 @@ class StoreDomain(Base, TimestampMixin):
     is_verified = Column(Boolean, default=False, nullable=False)
     verified_at = Column(DateTime(timezone=True), nullable=True)
 
+    # Platform association (for platform context resolution from custom domains)
+    platform_id = Column(
+        Integer,
+        ForeignKey("platforms.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+        comment="Platform this domain is associated with (for platform context resolution)",
+    )
+
     # Relationships
     store = relationship("Store", back_populates="domains")
+    platform = relationship("Platform")
 
     # Constraints
     __table_args__ = (
         UniqueConstraint("store_id", "domain", name="uq_store_domain"),
         Index("idx_domain_active", "domain", "is_active"),
         Index("idx_store_domain_primary", "store_id", "is_primary"),
+        Index("idx_store_domain_platform", "platform_id"),
     )
 
     def __repr__(self):