feat: implement configurable background jobs and toggle flags

Introduce environment variables to globally and individually control background jobs:
- `WARPBOX_JOBS_ENABLED` to toggle all background workers.
- `WARPBOX_CLEANUP_ENABLED` to toggle the expired box cleanup job.
- `WARPBOX_THUMBNAIL_ENABLED` to toggle the thumbnail generation job.

Refactor background tasks into a dedicated `backend/libs/jobs` package, allowing jobs to be registered, scheduled, and conditionally run based on the new configuration flags. Additionally, update the default maximum upload size in `.env.example` to 16GB and document the new settings in the README.

backend/libs/config/config_test.go

@@ -32,3 +32,20 @@ func TestParseMegabytesRejectsInvalidValues(t *testing.T) {
 		}
 	}
 }
+
+func TestEnvBool(t *testing.T) {
+	t.Setenv("WARPBOX_TEST_BOOL", "false")
+	if got := envBool("WARPBOX_TEST_BOOL", true); got {
+		t.Fatalf("envBool() = true, want false")
+	}
+
+	t.Setenv("WARPBOX_TEST_BOOL", "1")
+	if got := envBool("WARPBOX_TEST_BOOL", false); !got {
+		t.Fatalf("envBool() = false, want true")
+	}
+
+	t.Setenv("WARPBOX_TEST_BOOL", "not-a-bool")
+	if got := envBool("WARPBOX_TEST_BOOL", true); !got {
+		t.Fatalf("envBool() did not fall back to true")
+	}
+}