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/users.go

@@ -19,6 +19,12 @@ type scanner interface {
 	Scan(dest ...any) error
 }
 
+// queryer is the read surface shared by *sql.DB and *sql.Tx, so a list helper can
+// serve both a standalone read and one inside a transaction.
+type queryer interface {
+	QueryContext(ctx context.Context, query string, args ...any) (*sql.Rows, error)
+}
+
 // scanUser reads one users row. is_admin is stored as INTEGER 0/1 (the driver
 // returns it as int64, which does not convert straight to bool), so it is read
 // into an int64 and mapped.