feat: redesign crawler metrics figures with per-crawl bars on a shared time axis

[parismic]

Supersedes #50, which was merged prematurely and has been reverted from master - all review discussion, Sebastian's change requests, and the approvals live there.

• CrawlDb status and fetch status figures: vertical stacked bars over a datetime x-axis (derived from the crawl label), one fixed-width bar (21 days, the shortest crawl interval) per crawl, single crawls are distinguishable and the irregular intervals between crawls show up as gaps
• figures keep a fixed landscape size instead of growing with every crawl; both time-axis figures share the same x-range and axes geometry
• combined crawler metrics figure: unchanged standard dot/line plot, consistent with the other plots on the page (the mixed log-linear scale from the first iteration is dropped)
• stronger CrawlDb status colors; vertical grid lines removed
• rewrite plots/crawlermetrics.md as a funnel narrative, document the relation between fetch list, fetch total and the status counts, and explain the success-rate history (outdated seed donations until 2016; robots.txt/HTTP-403 denials and the 2022 exponential backoff recently

[sebastian-nagel]

Thanks, @parismic. Looks good.

There is a merge conflict related to #51. Likely this requires some additional work to make the new methods write the CSV files.