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/schemas/store_domain.py

@@ -28,6 +28,7 @@ class StoreDomainCreate(BaseModel):
     is_primary: bool = Field(
         default=False, description="Set as primary domain for the store"
     )
+    platform_id: int | None = Field(None, description="Platform this domain belongs to")
 
     @field_validator("domain")
     @classmethod
@@ -86,6 +87,7 @@ class StoreDomainResponse(BaseModel):
     is_active: bool
     is_verified: bool
     ssl_status: str
+    platform_id: int | None = None
     verification_token: str | None = None
     verified_at: datetime | None = None
     ssl_verified_at: datetime | None = None