Add plantings backend: plop CRUD, derived counts, removed_at (#14)

- domain: Planting gains a computed (non-persisted) DerivedCount field.
- store/plantings.go: Get/Create/Update (version-guarded)/Delete alongside the
  existing /full read helper.
- service/plantings.go: place/move/resize/soft-remove a plop; editor role on the
  object's garden; object must be plantable; plant_id must be visible to the
  actor (built-in or own) else ErrInvalidInput; center must sit within the
  object's unrotated local bounds (radius may overhang); planted_at defaults to
  today. derivedCount = max(1, round(π·r²/spacing²)) — one unit-tested helper,
  reused by /full (via a spacing map, no N+1) and single responses.
- api: POST /objects/:id/plantings, PATCH/DELETE /plantings/:id; nullable
  count/label/plantedAt/removedAt use RawMessage so null (clear) is distinct
  from absent (unchanged). removedAt is the soft-remove / "clear bed" seam.
- /full now enriches each active plop with its derivedCount.

Service tests: formula edge cases (tiny radius → 1), defaults + derived, count
override, move/resize + clear override, non-plantable rejection, foreign/unknown
plant rejection, bounds, soft-remove leaves /full, version conflict, cross-user
masking, delete. Plus an API-level create/patch/full/delete flow.

GOWORK=off go build/vet/test ./internal/... green.

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

internal/domain/domain.go

@@ -186,4 +186,10 @@ type Planting struct {
 	Version   int64   `json:"version"`
 	CreatedAt string  `json:"createdAt"`
 	UpdatedAt string  `json:"updatedAt"`
+
+	// DerivedCount is the plant count implied by the plop's area and the plant's
+	// mature spacing: max(1, round(π·r² / spacing²)). It is computed by the
+	// service, never persisted (the store scan leaves it 0). The effective count
+	// a client shows is Count when non-nil, else DerivedCount.
+	DerivedCount int `json:"derivedCount"`
 }