Add local token usage dashboard with SQLite storage

[tdu-naifen]

Track per-request input/output token usage in a local SQLite database and expose a live dashboard at /dashboard on the running server.

Features:

• SQLite store (bun:sqlite) at ~/.local/share/copilot-api/token-usage.db
  • 30-day auto-prune, 1-minute bucket resolution
  • Atomic upsert per (timestamp_min, model) composite key
• Token capture for both /chat/completions and /v1/messages endpoints
  • Non-streaming: reads response.usage from Copilot API response
  • Streaming: injects stream_options.include_usage=true so Copilot returns usage in the final SSE chunk; falls back to local tokenizer estimate
• GET /token-usage?range=1h|6h|24h|7d|30d JSON API
• GET /dashboard — shadcn-inspired dark UI with:
  • Stat cards (total requests, input tokens, output tokens)
  • Area chart (Chart.js via CDN, input vs output over time)
  • Per-model breakdown table
  • Copilot Budget section: fetches /usage and renders progress bars for premium_interactions, chat, completions quota with color thresholds (green <80%, yellow 80-95%, red >=95%, gray for unlimited)
  • 30-second auto-refresh
• Unit tests for budget color/percent helpers (9 tests)