feat: Cost tracking — per-session token usage and cost estimation

[OneStepAt4time]

Motivation

Competitor claudio tracks token usage and API costs per session with configurable limits. This is valuable for:

• Budget control in team environments
• Understanding which tasks are most expensive
• Optimizing prompt engineering

Proposal

• Track input/output tokens per session from CC JSONL events
• Estimate cost based on model pricing
• Expose via GET /v1/sessions/:id/metrics (already partially exists)
• Add GET /v1/metrics for aggregate stats
• Dashboard: cost column in session list

Source

CC JSONL events include token usage. Parse and aggregate.
Competitor: https://github.com/Iron-Ham/claudio

[OneStepAt4time]

Need to be analyzed in depth