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/server/image_proxy_internal_test.go

@@ -113,7 +113,7 @@ func TestImageProxyFlow(t *testing.T) {
 			}
 
 			checkSourceCached(t, db, stateDir, source)
-			checkVariantCached(t, db, stateDir, first.Body.Bytes(), tt.contentType)
+			checkVariantCached(t, db, stateDir, first.Body.Bytes())
 		})
 	}
 }
@@ -276,27 +276,19 @@ func checkSourceCached(t *testing.T, db *sql.DB, stateDir string, source []byte)
 }
 
 // checkVariantCached checks that the converted image served is recorded in
-// variant_content with contentType, and stored under its cache key in
-// cache/variants.
-func checkVariantCached(
-	t *testing.T, db *sql.DB, stateDir string, served []byte, contentType string,
-) {
+// variant_content, and stored under its cache key in cache/variants.
+func checkVariantCached(t *testing.T, db *sql.DB, stateDir string, served []byte) {
 	t.Helper()
 
-	var cacheKey, storedType string
+	var cacheKey string
 
 	err := db.QueryRowContext(t.Context(),
-		"SELECT cache_key, content_type FROM variant_content",
-	).Scan(&cacheKey, &storedType)
+		"SELECT cache_key FROM variant_content",
+	).Scan(&cacheKey)
 	if err != nil {
 		t.Fatalf("variant_content row: %v", err)
 	}
 
-	if storedType != contentType {
-		t.Errorf("variant_content content_type = %q, want %q",
-			storedType, contentType)
-	}
-
 	checkFile(t, filepath.Join(stateDir, "cache", "variants",
 		cacheKey[0:2], cacheKey[2:4], cacheKey), served)
 }