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/data/variety_repository.dart

@@ -1987,6 +1987,22 @@ class VarietyRepository {
     return id;
   }
 
+  /// The id of the non-deleted variety whose label matches [label]
+  /// (case-insensitive, trimmed), or null. This is how a scanned envelope
+  /// label finds its record: the label IS the identity a keeper printed.
+  Future<String?> findVarietyIdByLabel(String label) async {
+    final row =
+        await (_db.select(_db.varieties)
+              ..where(
+                (v) =>
+                    v.isDeleted.equals(false) &
+                    v.label.lower().equals(label.trim().toLowerCase()),
+              )
+              ..limit(1))
+            .getSingleOrNull();
+    return row?.id;
+  }
+
   /// Records how a season went for a lot — the optional, skippable answer to
   /// "how did it do?" asked when a harvest is noted. Returns the new row id.
   Future<String> addGardenOutcome({