Keep application/octet-stream out of memory for variants (closes #70)

VariantStorage.LoadWithMeta now returns an empty content type when the
.meta file is missing or unreadable, and GetVariant serves
application/octet-stream in that case without keeping it in memory.
Memory now only ever holds a content type read from a .meta file or
passed to StoreVariant, so a read that ran before a store wrote the
.meta file can no longer leave the wrong type in memory, however it
interleaves with the store, other reads and eviction.

Model: opus-5-5

internal/imgcache/cache.go

@@ -184,7 +184,9 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 
 // GetVariant returns a reader, size, and content type for a cached
 // variant. The content type comes from metaCache, or else from the
-// variant's .meta file and is then kept in metaCache.
+// variant's .meta file and is then kept in metaCache. A variant with
+// no .meta file is served as application/octet-stream, which is not
+// kept.
 func (c *Cache) GetVariant(cacheKey VariantKey) (io.ReadCloser, int64, string, error) {
 	if c.disabled {
 		return nil, 0, "", ErrNotFound
@@ -524,9 +526,11 @@ func (c *Cache) IncrementTransformCount(ctx context.Context) {
 // loadVariantWithMeta is GetVariant for a variant metaCache does not
 // hold: it reads the content type from the variant's .meta file and
 // keeps it in metaCache, unless a StoreVariant has put one there
-// meanwhile. The stored one wins, since this read may have found the
-// variant file before the store wrote the .meta file, and so got
-// application/octet-stream.
+// meanwhile, as the store's is newer. A read that finds no .meta file,
+// as one can between a store's writing of the variant file and of its
+// .meta file, serves application/octet-stream and keeps nothing, so
+// metaCache only ever holds a type read from a .meta file or passed to
+// StoreVariant.
 func (c *Cache) loadVariantWithMeta(
 	cacheKey VariantKey,
 ) (io.ReadCloser, int64, string, error) {
@@ -535,6 +539,10 @@ func (c *Cache) loadVariantWithMeta(
 		return nil, 0, "", err
 	}
 
+	if contentType == "" {
+		return reader, size, fallbackContentType, nil
+	}
+
 	c.metaCache.ContainsOrAdd(cacheKey, contentType)
 
 	return reader, size, contentType, nil