Copy a garden (deep duplicate) from the gardens list

Adds POST /gardens/:id/copy: duplicates a garden the actor owns, along
with its objects and their currently-planted plops, into a new garden
owned by the actor. A blank name derives "<source> (copy)".

Deliberately not carried over:
  * public_token — the share link is a capability granted to the
    original; a copy must not silently inherit a live public URL
  * garden_shares — a copy is private to whoever made it
  * removed plantings — the copy is a fresh layout, not a history

Owner-only for now: a copy's plops keep pointing at the SOURCE's
plant_ids, so letting a viewer copy someone else's garden would hand
them plants outside their catalog and pin the original owner's plants
against deletion (plantings.plant_id is ON DELETE RESTRICT). Copying a
shared garden needs a plant-cloning policy first.

The whole copy runs in one transaction, so a failure part-way leaves no
half-populated garden. The object/planting list scans are factored into
queryObjects/queryPlantings so the same code serves a plain read and a
read inside that transaction.

UI: a Copy action on the owner's garden card opens a modal prefilled
with the derived name, then lands in the new garden.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

internal/store/objects.go

@@ -66,10 +66,17 @@ func (d *DB) GetObject(ctx context.Context, id int64) (*domain.GardenObject, err
 // ListObjectsForGarden returns a garden's objects, bottom-to-top (z_index then
 // id). Always a non-nil slice.
 func (d *DB) ListObjectsForGarden(ctx context.Context, gardenID int64) ([]domain.GardenObject, error) {
-	rows, err := d.sql.QueryContext(ctx,
+	return queryObjects(ctx, d.sql,
 		`SELECT `+objectColumns+` FROM garden_objects WHERE garden_id = ? ORDER BY z_index, id`,
-		gardenID,
-	)
+		gardenID)
+}
+
+// queryObjects runs an object query and scans every row. The result set is fully
+// drained and the cursor closed before returning, so a caller inside a
+// transaction may write on the same connection afterwards. Always a non-nil
+// slice.
+func queryObjects(ctx context.Context, q queryer, query string, args ...any) ([]domain.GardenObject, error) {
+	rows, err := q.QueryContext(ctx, query, args...)
 	if err != nil {
 		return nil, fmt.Errorf("store: list objects: %w", err)
 	}