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

Co-authored-by: Steve Dudenhoeffer <[email protected]>

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"`
 }