Declare only the tables and columns pixa writes and reads (closes #310)

Removes output_content and request_cache, which nothing writes, and every
column that was written but never read: the host, path, query and
fetched_at of source_content, the content_type and created_at of
variant_content, and the status_code, error_message and fetched_at of
negative_cache.

StoreVariant no longer takes a content type and StoreNegative no longer
takes a status code or message, so extractStatusCode and the status
constants, which only fed status_code, are gone. Reconciliation adopts a
variant file without opening it, as its row no longer records a content
type. Tests that set or checked a removed column follow.

Model: opus-5-5

internal/imgcache/testutil_internal_test.go

@@ -24,7 +24,6 @@ const (
 	testHostExample       = "example.com"
 	testPathCat           = "/photos/cat.jpg"
 	testContentTypeJPEG   = "image/jpeg"
-	testContentTypeWebP   = "image/webp"
 	testHeaderContentType = "Content-Type"
 )