Address Gadfly review on #7: garden cap, dim validation, shared errors

Fixes from the PR #26 adversarial review (graded 18 real / 0 false positive).

Correctness / security
- maxGardenCM fixed to 10_000 (100 m), matching its comment — it was
  100_000 cm (1 km), 10x too lax (5 models flagged this).
- Dimension validation now rejects NaN/Inf (which slip past naive
  comparisons) and subnormal-tiny positives, via a finite [1cm, 100m]
  check. Name (200) and notes (10_000) are length-capped so untrusted
  input can't balloon storage.
- Update version binding is `required,min=1`, so a negative/zero version
  is a 400, not a 409.

Maintainability / performance
- One unified writeServiceError (new errors.go) maps every auth + resource
  sentinel; writeResourceError removed. writeVersionConflict and
  parseIDParam moved to errors.go (shared, not in the gardens feature file).
- Request structs share an embedded gardenFields (one toInput).
- CreateGarden uses INSERT ... RETURNING (one round-trip).
- ListGardensForOwner has a defensive LIMIT (pagination is post-v1).

Tests: name/notes length, NaN/Inf/subnormal dims, dimension-at-cap valid,
negative/zero version -> 400.

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

internal/store/gardens.go

@@ -23,22 +23,24 @@ func scanGarden(s scanner) (*domain.Garden, error) {
 	return &g, nil
 }
 
+// maxGardensListed caps ListGardensForOwner as a defensive backstop against a
+// pathologically large result. At household scale a user has a handful of
+// gardens, so this is never hit; genuine pagination is post-v1 if ever needed.
+const maxGardensListed = 1000
+
 // CreateGarden inserts a garden (owner_id, name, dimensions, unit, notes already
 // set and validated by the service) and returns the stored row.
 func (d *DB) CreateGarden(ctx context.Context, g *domain.Garden) (*domain.Garden, error) {
-	res, err := d.sql.ExecContext(ctx,
+	created, err := scanGarden(d.sql.QueryRowContext(ctx,
 		`INSERT INTO gardens (owner_id, name, width_cm, height_cm, unit_pref, notes)
-		 VALUES (?, ?, ?, ?, ?, ?)`,
+		 VALUES (?, ?, ?, ?, ?, ?)
+		 RETURNING `+gardenColumns,
 		g.OwnerID, g.Name, g.WidthCM, g.HeightCM, g.UnitPref, g.Notes,
-	)
+	))
 	if err != nil {
 		return nil, fmt.Errorf("store: insert garden: %w", err)
 	}
-	id, err := res.LastInsertId()
-	if err != nil {
-		return nil, fmt.Errorf("store: garden insert id: %w", err)
-	}
-	return d.GetGarden(ctx, id)
+	return created, nil
 }
 
 // GetGarden returns the garden with the given id, or domain.ErrNotFound.
@@ -59,8 +61,8 @@ func (d *DB) GetGarden(ctx context.Context, id int64) (*domain.Garden, error) {
 // added in #16.
 func (d *DB) ListGardensForOwner(ctx context.Context, ownerID int64) ([]domain.Garden, error) {
 	rows, err := d.sql.QueryContext(ctx,
-		`SELECT `+gardenColumns+` FROM gardens WHERE owner_id = ? ORDER BY created_at DESC, id DESC`,
-		ownerID,
+		`SELECT `+gardenColumns+` FROM gardens WHERE owner_id = ? ORDER BY created_at DESC, id DESC LIMIT ?`,
+		ownerID, maxGardensListed,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("store: list gardens: %w", err)