[FEATURE] Add Cost Attribution Tracking

[JoshuaAFerguson]

Track costs per user/team for chargeback.

Cost Metrics:

• Session minutes
• CPU cost
• Memory cost
• Storage cost
• Total cost per user

Implementation:

type CostAttribution struct {
    UserID         string
    SessionMinutes int
    CPUCost        float64
    MemoryCost     float64
    StorageCost    float64
    TotalCost      float64
}

Acceptance Criteria:

• Cost tracked per user/team
• Monthly cost reports
• API endpoint for costs
• Export to CSV

Files: api/internal/billing/cost_attribution.go (NEW)