Keep variant content types in memory for cache hits (closes #70)

Cache.metaCache was declared and never used, so every cache hit read
and parsed the variant's .meta file. It is now an LRU
(github.com/hashicorp/golang-lru/v2) of up to 10,000 variants' content
types, filled by StoreVariant and by a read of a .meta file, so a hit
for a variant it holds skips the .meta read. Only a type from a .meta
file or a store ever enters memory, never the application/octet-stream
fallback, and a stored type is never replaced by an older one from
disk. The variant file itself is still opened on every hit, so nothing
is served from memory alone.

Model: opus-5-5

TODO.md

@@ -29,6 +29,17 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 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, where a type `StoreVariant` added
+  meanwhile is kept over the one read, and never with the
+  `application/octet-stream` served for a variant without one; for a variant it
+  holds, `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 will not open; the cap is a
+  constant, not a setting; the unused `variantMeta` type is gone; `README.md`
+  describes it.
 - 2026-09-29 maintenance mode refuses image requests (closes #71): while
   `maintenance_mode` is on, `/v1/image/` and `/v1/e/` answer 503 with a
   `Retry-After` header and the JSON error body, from one middleware in