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/source_content_internal_test.go

@@ -32,10 +32,8 @@ func TestStoreSource_StoresIdenticalBytesOfTwoURLsTwice(t *testing.T) {
 	}
 
 	for sourcePath, key := range keys {
-		rows := countRows(t, cache, `
-			SELECT COUNT(*) FROM source_content
-			WHERE source_key = ? AND source_host = ? AND source_path = ?
-		`, string(key), testHostCDN, sourcePath)
+		rows := countRows(t, cache,
+			`SELECT COUNT(*) FROM source_content WHERE source_key = ?`, string(key))
 		if rows != 1 {
 			t.Errorf("%s has %d source_content rows, want 1", sourcePath, rows)
 		}