Seed provenance + inventory: source links and seed lots (#50) (#64)

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

internal/api/errors.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"encoding/json"
 	"errors"
 	"log/slog"
 	"net/http"
@@ -69,6 +70,26 @@ func writeVersionConflict(c *gin.Context, current any) {
 	})
 }
 
+// parseNullable decodes any JSON value into (value, present) for a nullable
+// column: absent → (nil, false); explicit null → (nil, true); anything else →
+// the decoded value. That three-way distinction is what lets a PATCH tell "clear
+// this to NULL" apart from "leave it alone", and every nullable field in the API
+// needs it, so it lives here rather than in whichever file happened to want it
+// first.
+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
+}
+
 // intQuery reads a non-negative integer query parameter, falling back to def on
 // an absent or malformed value.
 func intQuery(c *gin.Context, name string, def int) int {