Keep a variant's accounting row when its file cannot be read (closes #340)

The reconciliation pass dropped a variant's accounting row on any error
reading its file's size, and logged every such file as missing. A file
still on disk then had no row, so eviction no longer counted it against
cache_max_bytes. Now only a missing file (ErrNotFound, which
FileStore.Size returns for it) loses its row, with the same info line.
Any other error keeps the row and logs a warning naming the variant key
and the error, and the pass goes on to the next key. README.md says so
under Storage.

Model: opus-5-5

internal/imgcache/eviction.go

@@ -613,7 +613,9 @@ func (c *Cache) adoptVariantFile(
 }
 
 // reconcileVariantRows drops accounting rows whose variant files are
-// missing, so the database never references deleted content.
+// missing, so the database never references deleted content. A file whose
+// size cannot be read for another reason may still be on disk, so its row
+// stays, with a warning naming its key and the error.
 func (c *Cache) reconcileVariantRows(ctx context.Context) error {
 	var after imagerequest.VariantKey
 
@@ -641,6 +643,13 @@ func (c *Cache) reconcileVariantRows(ctx context.Context) error {
 				continue
 			}
 
+			if !errors.Is(err, ErrNotFound) {
+				c.log.Warn("kept accounting row for unreadable variant file",
+					"cache_key", key, "error", err)
+
+				continue
+			}
+
 			_, err = c.db.ExecContext(ctx,
 				`DELETE FROM variant_content WHERE cache_key = ?`, string(key))
 			if err != nil {