Use objectPlantingsPath helper instead of inline URL (#88)

Gadfly: I was wrong that no plantings-path helper existed — objectPlantingsPath
is defined in plantings_test.go in the same package. Use it for the two
planting POSTs in the remaining test.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/api/seed_lots_test.go

@@ -163,7 +163,7 @@ func TestSeedLotRemainingIsDerivedAPI(t *testing.T) {
 	oid := int64(decodeMap(t, w.Body.Bytes())["id"].(float64))
 
 	// Plant 12 of them against the lot.
-	w = doJSON(t, r, http.MethodPost, objectPath(oid)+"/plantings", map[string]any{
+	w = doJSON(t, r, http.MethodPost, objectPlantingsPath(oid), map[string]any{
 		"plantId": plantID, "xCm": 0, "yCm": 0, "radiusCm": 20, "count": 12, "seedLotId": lotID,
 	}, cookie)
 	if w.Code != http.StatusCreated {
@@ -183,7 +183,7 @@ func TestSeedLotRemainingIsDerivedAPI(t *testing.T) {
 	// NEGATIVE. That's deliberate — the number is a derived truth about what
 	// you've committed, not a floor clamped at zero, and "you've planted more than
 	// you bought" is exactly the signal a gardener wants rather than a hidden -7.
-	w = doJSON(t, r, http.MethodPost, objectPath(oid)+"/plantings", map[string]any{
+	w = doJSON(t, r, http.MethodPost, objectPlantingsPath(oid), map[string]any{
 		"plantId": plantID, "xCm": 40, "yCm": 40, "radiusCm": 20, "count": 45, "seedLotId": lotID,
 	}, cookie)
 	if w.Code != http.StatusCreated {