[Task 16] Dashboard upgrade

[lubobill1990]

Task 16 — Dashboard upgrade

Depends on: 11, 13

Goal

pages/index.html shows the new stats payload. New sub-page for the price
timeline.

Scope

pages/index.html:

• Lens toggle (Historical / Current / Timeline) — controls ?lens= on every
  fetch.
• Currency display (stats.currency).
• Four cards: total cost, total requests, total tokens, active accounts.
• Account × Model matrix table (cell shows tokens above, cost below).
• Daily cost stacked bar chart (color = account).
• Daily token line chart (lines = input / cached / output / reasoning).
• Missing-pricing list.

New file pages/timeline.html:

• Per-model step chart of input_per_mtok, output_per_mtok,
  cached_input_per_mtok, reasoning_per_mtok over time.
• Hover reveals sync_log_id and a JSON diff popover.
• New /usage/timeline route returns
  model_pricing_versions rows joined with pricing_sync_log.

Use Chart.js via CDN. No bundler.

Definition of Done

• Three lens toggles visibly change displayed totals.
• Account × Model matrix correctly attributes tokens & cost.
• Daily charts render with at least one day of data.
• Timeline page shows a step-change at a synced price update.
• Page works against a fresh DB with zero data (empty states render
  cleanly).

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• Source: docs/tasks/16-dashboard.md
• Design: docs/design/
• Phase: ui
• Depends on tasks: 11, 13