Address Gadfly review on #10: clearable color/props, column lists, dedup

Fixes from the PR #28 adversarial review (considered; not graded).

Correctness / API
- PATCH /objects/:id can now clear nullable color/props back to NULL: the
  request takes them as json.RawMessage, and ObjectPatch carries an explicit
  Set flag so an explicit `null` (clear) is distinguished from an absent
  field (unchanged) — the strongest cross-model finding (6 hits). New test.

Maintainability
- store/plantings.go + plants.go use explicit qualified column lists
  (qualifyColumns helper) instead of SELECT *, matching gardens/objects and
  surviving a future column add.
- Consolidated objectKinds + plantableByDefault into one kind→traits map.
- objectForRole factors the fetch-then-authorize shared by UpdateObject and
  DeleteObject; dropped the redundant kind check in CreateObject
  (finalizeObject is the single validation point).
- Request→service mapping via toInput()/toPatch() methods (matches gardens).
- Renamed handler gardenFull → getGardenFull (verbNoun); test helper
  decodeGarden → decodeMap; generic bind-error messages.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/store/plantings.go

@@ -7,9 +7,11 @@ import (
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
 )
 
-// scanPlanting reads a plantings row in table-declared column order (id,
-// object_id, plant_id, x_cm, y_cm, radius_cm, count, label, planted_at,
-// removed_at, version, created_at, updated_at) — the order SELECT pl.* yields.
+// plantingColumns lists plantings columns in the order scanPlanting expects.
+// Used unqualified for direct selects; the /full read below qualifies with pl.
+const plantingColumns = `id, object_id, plant_id, x_cm, y_cm, radius_cm, count, label,
+	planted_at, removed_at, version, created_at, updated_at`
+
 func scanPlanting(s scanner) (*domain.Planting, error) {
 	var p domain.Planting
 	if err := s.Scan(
@@ -27,10 +29,8 @@ func scanPlanting(s scanner) (*domain.Planting, error) {
 // non-nil slice (empty until plantings CRUD lands in #14). Plop CRUD itself is
 // #14; this is only the /full read side.
 func (d *DB) ListActivePlantingsForGarden(ctx context.Context, gardenID int64) ([]domain.Planting, error) {
-	// pl.* returns the plantings columns in table-declared order, which matches
-	// plantingColumns / scanPlanting.
 	rows, err := d.sql.QueryContext(ctx,
-		`SELECT pl.* FROM plantings pl
+		`SELECT `+qualifyColumns("pl", plantingColumns)+` FROM plantings pl
 		   JOIN garden_objects o ON o.id = pl.object_id
 		  WHERE o.garden_id = ? AND pl.removed_at IS NULL
 		  ORDER BY pl.id`,