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/cachesize_internal_test.go

@@ -39,9 +39,8 @@ func TestComputeDefaultMaxBytesCountsWhatTheCacheHolds(t *testing.T) {
 
 	// The cache now holds those 3 GiB, which leaves 1 GiB free.
 	_, err = cache.db.ExecContext(t.Context(),
-		`INSERT INTO variant_content (cache_key, size_bytes, content_type)
-		 VALUES (?, ?, ?)`,
-		string(testVariantKeyOne), 3221225472, testContentTypeWebP,
+		`INSERT INTO variant_content (cache_key, size_bytes) VALUES (?, ?)`,
+		string(testVariantKeyOne), 3221225472,
 	)
 	if err != nil {
 		t.Fatalf("failed to insert variant accounting row: %v", err)