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, content_type
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.

StoreSource no longer takes the fetch result and LookupSource returns only
the source key. 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/database/concurrent_writes_internal_test.go

@@ -131,14 +131,13 @@ func writeLikeOneRequest(ctx context.Context, db *sql.DB, key string) error {
 	}
 
 	_, err = db.ExecContext(ctx, `INSERT INTO source_content
-		(source_key, source_host, source_path, content_type, size_bytes)
-		VALUES (?, 'example.com', '/image.png', 'image/png', 1)`, key)
+		(source_key, size_bytes) VALUES (?, 1)`, key)
 	if err != nil {
 		return fmt.Errorf("storing a source: %w", err)
 	}
 
 	_, err = db.ExecContext(ctx, `INSERT INTO variant_content
-		(cache_key, size_bytes, content_type) VALUES (?, 1, 'image/png')`, key)
+		(cache_key, size_bytes) VALUES (?, 1)`, key)
 	if err != nil {
 		return fmt.Errorf("recording a transformed image: %w", err)
 	}