Keep the content type a store put in memory over one read from disk (closes #70)

GetVariant now adds the content type it read from a .meta file only
when memory holds none for the variant, so a type StoreVariant added
meanwhile is not replaced. A read that found no .meta file yet still
gets application/octet-stream for that one request, as before this
change, but no longer leaves it in memory for later hits.

Model: opus-5-5

TODO.md

@@ -32,7 +32,8 @@ P2: security: referer blacklist
 - 2026-09-29 variant content types kept in memory (closes #70):
   `Cache.metaCache` holds the content types of up to 10,000 variants in an LRU
   (`github.com/hashicorp/golang-lru/v2`), filled by `StoreVariant` and by
-  `GetVariant` after it reads a `.meta` file; for a variant it holds, `Lookup`
+  `GetVariant` after it reads a `.meta` file, where a type `StoreVariant` added
+  meanwhile is kept over the one read; for a variant it holds, `Lookup`
   skips the check of the disk and `GetVariant` skips the `.meta` read, still
   opening the variant file and taking the size from it; eviction removes the
   entry before deleting the files, and `GetVariant` removes it when the file