feat(app): scan a printed seed label back into its record

Closes the physical↔digital loop the labels opened: the QR on a printed
envelope (tane://seed, already encoded by seed_label_codec) can now be
scanned from the inventory bar. A known label opens its variety; an
unknown one asks before adding anything, then creates the variety (species
linked by exact scientific name when bundled) with a lot carrying the
label's year and origin. Scanner is pure ZXing (zxing_barcode_scanner,
MIT) — no ML Kit, no Play Services, F-Droid-safe, the stack Ğ1nkgo ships.
Mobile-only; other platforms don't show the button (OCR precedent).
The post-scan flow is a plain function (handleScannedPayload) so it's
widget-tested without a camera.

apps/app_seeds/lib/services/seed_label_scan.dart

@@ -0,0 +1,66 @@
+import 'package:equatable/equatable.dart';
+
+import '../data/export_import/seed_label_codec.dart';
+import '../data/species_repository.dart';
+import '../data/variety_repository.dart';
+
+/// What scanning a QR yielded: not a seed label at all, a variety already in
+/// the collection ([varietyId] set), or a seed not held yet ([data] set — the
+/// UI asks before adding anything).
+class SeedScanResult extends Equatable {
+  const SeedScanResult.notALabel() : varietyId = null, data = null;
+  const SeedScanResult.match(String this.varietyId) : data = null;
+  const SeedScanResult.unknown(SeedLabelData this.data) : varietyId = null;
+
+  final String? varietyId;
+  final SeedLabelData? data;
+
+  @override
+  List<Object?> get props => [varietyId, data];
+}
+
+/// Resolves a scanned QR payload against the collection: decodes the
+/// `tane://seed` label and looks the variety up by its label (the identity a
+/// keeper printed on the envelope). This is how a physical packet finds its
+/// way back to its record — even a re-saved one whose digital thread broke.
+Future<SeedScanResult> resolveScannedLabel(
+  VarietyRepository repository,
+  String raw,
+) async {
+  final data = SeedLabelCodec.decode(raw);
+  if (data == null) return const SeedScanResult.notALabel();
+  final varietyId = await repository.findVarietyIdByLabel(data.varietyLabel);
+  return varietyId == null
+      ? SeedScanResult.unknown(data)
+      : SeedScanResult.match(varietyId);
+}
+
+/// Adds the scanned seed to the collection — a variety named as the label
+/// says, linked to the bundled species when the scientific name matches
+/// exactly, with one lot carrying the label's harvest year and origin.
+/// Called only after the person confirmed the add prompt.
+Future<String> importScannedLabel({
+  required VarietyRepository repository,
+  required SpeciesRepository species,
+  required SeedLabelData data,
+}) async {
+  final varietyId = await repository.addQuickVariety(label: data.varietyLabel);
+
+  final scientific = data.scientificName?.trim();
+  if (scientific != null && scientific.isNotEmpty) {
+    final matches = await species.search(scientific, limit: 4);
+    for (final m in matches) {
+      if (m.scientificName.toLowerCase() == scientific.toLowerCase()) {
+        await repository.linkSpecies(varietyId, m.id);
+        break;
+      }
+    }
+  }
+
+  await repository.addLot(
+    varietyId: varietyId,
+    harvestYear: data.year,
+    originName: data.origin,
+  );
+  return varietyId;
+}