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

README.md

@@ -188,10 +188,10 @@ allowlisted source hosts for open access.
 name.
 
 Each source image and each transformed image has one row in the database,
-holding its size and when it was last used. Two source URLs that serve the same
-bytes are stored twice, once under each URL. A source image is stored by writing
-its file and then its row; when the row cannot be written, the file is removed
-again.
+holding its size and when it was last used; a source image's row also holds its
+content type. Two source URLs that serve the same bytes are stored twice, once
+under each URL. A source image is stored by writing its file and then its row;
+when the row cannot be written, the file is removed again.
 
 Source and transformed image files are written under a temporary name, synced to
 disk, and only then renamed into place, so a crash leaves no file under its own