Have Lookup check the disk for variants held in memory too (closes #70)

Lookup again counts a variant as present only when its file exists.
Treating a variant held in memory as present saved one check of the
disk, but no test covered it, and GetVariant opens the file anyway.

Model: opus-5-5

TODO.md

@@ -33,12 +33,12 @@ P2: security: referer blacklist
   `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; 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
-  will not open; the cap is a constant, not a setting; the unused `variantMeta`
-  type is gone; `README.md` describes it.
+  meanwhile is kept over the one read; 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

internal/imgcache/cache.go

@@ -159,15 +159,13 @@ type LookupResult struct {
 	CacheStatus CacheStatus
 }
 
-// Lookup checks if a processed variant exists on disk: a variant held
-// in metaCache counts as present without a check of the disk. Hits
-// touch the variant's LRU timestamp; a disabled cache always misses.
+// Lookup checks if a processed variant exists on disk. Hits touch the
+// variant's LRU timestamp; a disabled cache always misses.
 func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, error) {
 	cacheKey := CacheKey(req)
 
-	// Check memory, then variant storage - no DB needed for cache hits
-	if !c.disabled &&
-		(c.metaCache.Contains(cacheKey) || c.variants.Exists(cacheKey)) {
+	// Check variant storage directly - no DB needed for cache hits
+	if !c.disabled && c.variants.Exists(cacheKey) {
 		c.touchVariant(ctx, cacheKey)
 
 		return &LookupResult{