perf(inventory): lazy list, photo thumbnails, indexes, debounced reload

Scale the local inventory to 10k+ varieties:
- Render the list with ListView.builder over a flattened header/item model
  instead of building every tile upfront.
- Store a small regenerable JPEG thumbnail per photo (schema v14) and use it
  for the 48px list avatar; full bytes stay for offer image hosting. Existing
  photos are backfilled lazily at startup. Thumbnail is local-only (excluded
  from CRDT sync and backups by the JSON codec).
- Add indexes on varieties(is_deleted,is_draft), attachments(parent_type,
  parent_id,kind), lots(variety_id) via @TableIndex.
- Debounce watchInventoryView (~250ms) so a burst of table writes triggers one
  reload, not seven.
- cacheWidth/cacheHeight on the list avatar decode.
- Scale test raised 3k -> 10k; migration test v13 -> v14.

apps/app_seeds/lib/db/database.dart

@@ -32,7 +32,7 @@ class AppDatabase extends _$AppDatabase {
 
   /// Current schema version; also stamped into interchange exports so an
   /// importer knows which app generation wrote the file (data-model §7).
-  static const int currentSchemaVersion = 13;
+  static const int currentSchemaVersion = 14;
 
   @override
   int get schemaVersion => currentSchemaVersion;
@@ -196,9 +196,40 @@ class AppDatabase extends _$AppDatabase {
           await m.createTable(gardenOutcomes);
         }
       }
+      // v14: scalability for large inventories. A local, regenerable [thumbnail]
+      // BLOB on attachments (so the list never decodes full-resolution photos)
+      // plus indexes on the columns the inventory list filters/joins on. All
+      // additive and guarded, so a half-migrated dev database re-runs cleanly
+      // (see the v7 note above). Existing thumbnails are backfilled lazily in
+      // the background, not here (image decoding in a migration is slow/fragile).
+      if (from < 14) {
+        if (!await _hasColumn('attachments', 'thumbnail')) {
+          await m.addColumn(attachments, attachments.thumbnail);
+        }
+        // Declared as `@TableIndex` on the tables, so a fresh install gets them
+        // via createAll(); existing databases get them here. Guarded against a
+        // half-migrated dev database (see the v7 note above).
+        for (final index in [
+          idxVarietiesDeletedDraft,
+          idxAttachmentsParent,
+          idxLotsVariety,
+        ]) {
+          if (!await _hasIndex(index.entityName)) await m.create(index);
+        }
+      }
     },
   );
 
+  /// Whether an index named [name] already exists — keeps the additive v14
+  /// index creation idempotent against partially-migrated databases.
+  Future<bool> _hasIndex(String name) async {
+    final rows = await customSelect(
+      "SELECT 1 FROM sqlite_master WHERE type = 'index' AND name = ?",
+      variables: [Variable.withString(name)],
+    ).get();
+    return rows.isNotEmpty;
+  }
+
   /// Whether a table named [table] already exists. Keeps the additive v8
   /// table creation idempotent against partially-migrated databases.
   Future<bool> _hasTable(String table) async {