[SAAS-018] feat: Complete Sales Foundation module implementation
## Backend (NestJS)
- Entities: Lead, Opportunity, PipelineStage, Activity with TypeORM
- Services: LeadsService, OpportunitiesService, PipelineService, ActivitiesService, SalesDashboardService
- Controllers: LeadsController, OpportunitiesController, PipelineController, ActivitiesController, DashboardController
- DTOs: Full set of Create/Update/Convert DTOs with validation
- Tests: 5 test suites with comprehensive coverage
## Frontend (React)
- Pages: /sales, /sales/leads, /sales/leads/[id], /sales/opportunities, /sales/opportunities/[id], /sales/activities
- Components: SalesDashboard, ConversionFunnel, LeadsList, LeadForm, LeadCard, PipelineBoard, OpportunityCard, OpportunityForm, ActivityTimeline, ActivityForm
- Hooks: useLeads, useOpportunities, usePipeline, useActivities, useSalesDashboard
- Services: leads.api, opportunities.api, activities.api, pipeline.api, dashboard.api
## Documentation
- Updated SAAS-018-sales.md with implementation details
- Updated MASTER_INVENTORY.yml - status changed from specified to completed
Story Points: 21
Sprint: 6 - Sales Foundation
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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