Remove Workspace billing period fields

[BrewingCoder]

Parent: #25 — Remove all SaaS billing code

Depends on: #28 (pricing removal — these fields are read by pricing meter calculations)

What to Remove from Workspace struct

Field	Line	Type
BillingPeriodStart	275	*time.Time
BillingPeriodEnd	276	*time.Time
NextInvoiceDate	277	*time.Time

Callers

File	Line	Usage
private-graph/graph/resolver.go	499	Field mapping to response
private-graph/graph/resolver.go	515	NextInvoiceDate mapping
private-graph/graph/schema.resolvers.go	6341	BillingPeriodStart conditional check
pricing/pricing.go	467-480	Used as startDate/endDate in meter calculations
public-graph/graph/resolver_test.go	601-602, 612-613	Test data setup
worker/worker_test.go	911-912, 919-920	Test data setup

Migration Required

• Add DB migration to drop 3 columns from workspaces table

Test Plan

• Remove fields from struct
• Update resolver mappings
• Update tests that set these fields
• Add column-drop migration
• Verify build + tests

🤖 Generated with Claude Code

[github-actions]

AI Triage: Remove SaaS billing period fields (BillingPeriodStart, BillingPeriodEnd, NextInvoiceDate) from Workspace struct and database schema as part of billing code cleanup.

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