fix(tags): transaction on sync, scroll indicator, backend tests (37 tests)

- Wrap TagLinksService.sync() in db.transaction() to prevent race conditions
- Add CSS mask-image fade edges on TagStrip for scroll affordance
- Add 37 unit tests for tag controllers:
  - TagsController: 12 tests (CRUD, defaults, conflict, not-found)
  - TagGroupsController: 10 tests (CRUD, reorder, cascading)
  - TagLinksController: 15 tests (link/unlink, bulk, sync, query)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-ui/src/navigation/TagStrip.svelte

@@ -147,6 +147,21 @@
 		overflow-x: auto;
 		scrollbar-width: none;
 		-ms-overflow-style: none;
+		/* Fade edges to indicate scrollable content */
+		mask-image: linear-gradient(
+			to right,
+			transparent 0%,
+			black 2rem,
+			black calc(100% - 2rem),
+			transparent 100%
+		);
+		-webkit-mask-image: linear-gradient(
+			to right,
+			transparent 0%,
+			black 2rem,
+			black calc(100% - 2rem),
+			transparent 100%
+		);
 	}
 
 	.tag-strip-container::-webkit-scrollbar {