fix(admin): show cached input share using all input tokens

[rexdotsh]

Summary

• Compute the dashboard cached-input percentage as cacheReadTokens / inputTotalTokens, including cache writes in the denominator. The existing Claude display omitted cache writes and could round to 100% even with substantial writes.
• Label it cached input rather than cache hit: this is a token share, not a percentage of requests with a cache hit. Show - when input usage is unavailable instead of implying 0%.
• Cover both the overall KPI and provider breakdown in admin UI tests, including cache-write and no-usage cases.

Evidence

In a read-only 24-hour production bucket query on Sep 27, Opus 5.5 had ~245.4M cache-read, ~14.6M cache-write, and ~37.9K uncached input tokens: ~94.4% cached input, while the old dashboard rounded to 100%. This PR only changes display math; it does not alter stored token counts or proxy behavior.

Checks

• bun test: 143 passing
• bun lint
• bun typecheck

Independent of open PRs #49 and #52. Leave unmerged for review.