feat: per-request cost tracking (C7)

[MANVENDRA-github]

Attributes a USD cost to every trace, so "cost" is a real number instead of
an inferred request count. Surfaces spend, cache savings, and cost-over-time
on the dashboard. (Closes the cost gap from the v0.1.x audit.)

What changed

• New pure cost.ts (computeCostUsd): token usage × per-1K price → USD, or
  null when the model is unpriced or usage is unknown (never a guess).
• Config: optional pricing map (USD per 1K input/output tokens, per model)
  → ResolvedConfig.pricing; added to sentinel.config.example.json.
• Trace: costUsd on every record + span attribute sentinel.cost_usd,
  set on the real path (routed model) and on a cache hit (the spend the cache
  avoided). The SQLite store gains a cost_usd column with a guarded
  migration so pre-existing DBs upgrade in place.
• Dashboard: a "Cost" stat (with cache savings) and a cost-over-time
  sparkline; computeStats sums spend (non-cache) vs. saved (cache).

Tests

• cost math (null / free / float-dust rounding), config parse + negative
  reject, SQLite round-trip + migration, server attributes cost end-to-end,
  dashboard spend/saved aggregation, and an e2e cost assertion. Dashboard
  screenshot regenerated to show the new views.

How to verify

• pnpm verify → green (217 tests, coverage ≥90%)
• pnpm build → green
• pnpm test:e2e → green

No behavior change without a pricing block: cost stays null and nothing
else moves.