Remove SendBillingNotifications, BillingEmailHistory, and emailHistoryCache

[BrewingCoder]

Parent: #25 — Remove all SaaS billing code

What to Remove

• SendBillingNotifications() function — model/model.go:1995
• BillingEmailHistory struct — model/model.go:1356
• emailHistoryCache global var — model/model.go:40 (+ init at :216-218)
• BillingEmailHistory entry in Models slice — model/model.go:185
• TTL cache import: github.com/ReneKroon/ttlcache

Callers (must be updated/removed)

File	Line	Usage
public-graph/graph/resolver.go	692, 696	Calls for errors usage 80%/100%
public-graph/graph/resolver.go	1270, 1274	Calls for session usage 80%/100%
worker/kafka_worker.go	380	Calls in batch processing loop
private-graph/graph/resolver_test.go	1168-1201	Test creates BillingEmailHistory records

Migration Required

• Add DB migration to drop billing_email_histories table
• Drop unique index email_history_active_workspace_type_idx

Test Plan

• Remove function and struct
• Update callers to no-op or remove billing notification calls entirely
• Update/remove resolver_test.go test that creates BillingEmailHistory
• Add migration to drop table
• Verify go build ./... passes
• Verify go test ./... passes
• Run SonarQube scan

🤖 Generated with Claude Code

[github-actions]

AI Triage: Remove SaaS billing-related code (SendBillingNotifications function, BillingEmailHistory struct, and emailHistoryCache) and associated database migration as part of removing all billing functionality.

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