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]>

DESIGN.md

@@ -52,6 +52,7 @@ POST /auth/register | /auth/login | /auth/logout     GET /auth/me      GET /auth
 GET  /auth/oidc/login      GET /auth/oidc/callback   (authorization-code + PKCE)
 GET,POST /gardens                                    GET,PATCH,DELETE /gardens/:id
 GET  /gardens/:id/full        ← one-shot editor load: garden + objects + plantings + referenced plants
+POST /gardens/:id/copy        ← deep-copy a garden you own (objects + active plops; not shares/link)
 POST /gardens/:id/objects     PATCH,DELETE /objects/:id
 POST /objects/:id/plantings   PATCH,DELETE /plantings/:id
 GET,POST /plants              PATCH,DELETE /plants/:id            (own plants only)