Avoid duplicate package decodes for deprecation badges

[thomashoneyman]

When accessing any version of a package except the latest we are decoding it twice — once to figure out if it's deprecated, and again to render the docs.

With this change we only ever have to decode once, by:

• replace latest-package decoding in package page handlers with a version-stamped deprecation marker
• keep latest-version pages on the existing single-decode path
• treat missing, stale, malformed, or unreadable markers as cache misses and recompute

Manually verified with an old Deku version and the page cache cleared; without the marker we measure a 1.07s, and with the marker at 0.47s.