Address Gadfly review on seed lots

Three real bugs, two of which would have quietly corrupted inventory numbers.

CopyGarden carried seed_lot_id into the copied plantings, so copying a garden
double-counted the original lot's usage — the copy is a plan, not a second
planting out of the same packet. It also quietly attached a private lot to a
garden that may later be shared, contradicting the "lots are never shared with a
garden" invariant this feature was built on. The copy now drops the link.

RestorePlanting restored a seed_lot_id verbatim from a history snapshot. Live
rows get their link nulled by ON DELETE SET NULL when a lot is deleted, but the
snapshot still holds the old id, so undoing a planting deletion after retiring
its lot would violate the foreign key and abort the whole revert. The revert now
drops a dangling link before restoring: the plant really was in the ground,
which is the part worth restoring.

CreateSeedLot returned a lot with Used/Remaining at their zero values, so a
packet you just bought reported "0 remaining" — the exact opposite of the truth.
The version-conflict path had the same gap, and that row is what the client
rebases on, so a 409 reporting remaining=0 is worse than no number at all. Both
fill them now.

Remaining can go negative and that is deliberate — it means more was planted
than the lot recorded buying, which happens (a miscounted packet, a lot entered
after the fact). Clamping to zero would hide the discrepancy at exactly the
moment it is worth seeing. Now documented and tested rather than incidental.

Performance and tidiness: SeedLotUsage is scoped to the lots being filled
instead of scanning every planting the owner has, so reading one lot is one
lot's work; ListSeedLotsForOwner gained the LIMIT every other list read has;
parseNullable moved out of seed_lots.go into the shared request helpers, since
every nullable field in the API needs that three-way absent/null/value
distinction; finalizePlant validates against its own maxPlantVendorLen rather
than borrowing a constant named for seed lots; seedLotForPlanting became
checkSeedLotForPlanting, since it validates rather than fetches; and the
duplicate intPtr test helper gave way to the existing ptrInt.

Declined: recording seed-lot mutations in the revision history. change_sets are
anchored to a garden and lots are user-scoped, deliberately — they belong to
you, not to any one garden, so there is no garden to record them against.

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

internal/api/seed_lots.go

@@ -81,22 +81,6 @@ func (r seedLotUpdateRequest) toPatch() (service.SeedLotPatch, error) {
 	return p, nil
 }
 
-// parseNullable decodes any JSON value into (value, present) for a nullable
-// column. Absent → (nil, false); null → (nil, true); otherwise the decoded value.
-func parseNullable[T any](raw json.RawMessage) (*T, bool, error) {
-	if len(raw) == 0 {
-		return nil, false, nil
-	}
-	if string(raw) == "null" {
-		return nil, true, nil
-	}
-	var v T
-	if err := json.Unmarshal(raw, &v); err != nil {
-		return nil, false, err
-	}
-	return &v, true, nil
-}
-
 func (h *handlers) listSeedLots(c *gin.Context) {
 	var plantID *int64
 	if raw := c.Query("plantId"); raw != "" {