test: massive test coverage expansion (+1,437 tests, 22 new files)

Expand test coverage across all layers via 14 parallel agents:

Unit tests (3,378 total, +534):
- First-ever AI sidecar tests (157 tests covering bridge lifecycle, all 12 tool modules)
- API route infrastructure (auth, pipeline, batch, settings, teams, roles, audit, api-keys, files, docs)
- Lib coverage improvements (audit 7%->95%, worker-pool 33%->100%)
- Web store/lib gap fills (features-store, tool-registry)

Integration tests (4,403 total, +903):
- Expanded 19 tool test files with parameter variations, format edge cases, boundary values
- Cross-format matrix: 290 tests covering 14 tools x 17 formats
- Adversarial/edge cases: 63 tests for extreme inputs, concurrent requests, corrupted files

E2E-Docker (125 new tests):
- Expanded 8 spec files + 1 new file covering all 49 tools
- Added HEIC/format handling, auth failures, download verification

GUI E2E (expanded 28 spec files):
- Navigation, responsive layout, keyboard shortcuts
- All 51 tool UIs with settings, processing, display modes
- Batch/pipeline workflows, settings/RBAC, visual regression
- Resilience, accessibility (ARIA, contrast, focus), performance budgets

tests/unit/ai/background-removal.test.ts

@@ -357,7 +357,7 @@ describe("removeBackground", () => {
     });
 
     it("upscales mask back to original dimensions after processing", async () => {
-      const callCount = 0;
+      const _callCount = 0;
       const resizeFn = vi.fn().mockReturnThis();
       vi.mocked(sharp).mockImplementation(
         () =>
@@ -376,7 +376,7 @@ describe("removeBackground", () => {
           c[0] && typeof c[0] === "object" && (c[0] as Record<string, unknown>).width === 5000,
       );
       expect(upscaleCall).toBeDefined();
-      expect(upscaleCall![0]).toMatchObject({ width: 5000, height: 3000, fit: "fill" });
+      expect(upscaleCall?.[0]).toMatchObject({ width: 5000, height: 3000, fit: "fill" });
     });
 
     it("does not upscale mask when image was not downscaled", async () => {