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

internal/imgcache/cache.go

@@ -522,7 +522,10 @@ 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.
+// 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.
 func (c *Cache) loadVariantWithMeta(
 	cacheKey VariantKey,
 ) (io.ReadCloser, int64, string, error) {
@@ -531,7 +534,7 @@ func (c *Cache) loadVariantWithMeta(
 		return nil, 0, "", err
 	}
 
-	c.metaCache.Add(cacheKey, contentType)
+	c.metaCache.ContainsOrAdd(cacheKey, contentType)
 
 	return reader, size, contentType, nil
 }