Remove AWSMarketplaceCustomer model

[BrewingCoder]

Parent: #25 — Remove all SaaS billing code

What to Remove

• AWSMarketplaceCustomer struct — model/model.go:378-384
• AWSMarketplaceCustomer field on Workspace struct — model/model.go:272
• AWSMarketplaceCustomer entry in Models slice — model/model.go:134
• aws-sdk-go-v2/service/marketplaceentitlementservice import (already removed from go.mod by tidy)
• aws-sdk-go-v2/service/marketplacemetering import (already removed from go.mod by tidy)

Callers (must be updated/removed)

File	Line	Usage
private-graph/graph/resolver.go	499	Join in query builder
private-graph/graph/schema.resolvers.go	6509-6512	Null check + field access (CustomerIdentifier, CustomerAWSAccountID, ProductCode)
worker/worker_test.go	922	Test: instantiation in workspace fixture

Migration Required

• Add DB migration to drop aws_marketplace_customers table
• Remove foreign key reference from workspaces

Test Plan

• Remove struct and Workspace field
• Update resolvers to remove AWS marketplace references
• Update worker_test.go fixture
• Add migration to drop table
• Verify build + tests pass

🤖 Generated with Claude Code

[github-actions]

AI Triage: Remove AWS Marketplace billing integration code including model struct, resolver references, and database migration as part of SaaS billing cleanup.

Labels applied: area:backend, tech-debt, billing-removal, priority:medium, component:database