External API v2 parity for time chart display settings (legend, tooltip, line style)

[alex-fedotyev]

#2776 adds three per-tile display settings for line/area time charts (showLegend, tooltipMode, lineInterpolation) on SharedChartSettingsSchema. That PR is app-side only: the v2 external API maps tile config by explicit per-display-type construction in packages/api/src/routers/external-api/v2/utils/dashboards.ts, so the new fields do not reach the external contract or the generated OpenAPI.

This tracks the external API parity follow-up:

• Add the three fields to the v2 tile config mapping (the per-display-type construction in dashboards.ts) for the line display type.
• Mirror them in the external API Zod schema (packages/api/src/utils/zod.ts) and the hand-written OpenAPI JSDoc so the generated spec includes them.
• Add v2 round-trip tests (save then get) covering the three fields.
• Update the customer-facing dashboard tile docs for the new options.

Depends on #2776.