[Security]: RUSTSEC-2026-0187: lopdf 0.41.0 in Cargo.lock (stack overflow via deeply nested PDF objects)

### Summary

`cargo audit` reports a vulnerability in the locked `lopdf` dependency:
RUSTSEC-2026-0187 (stack overflow when parsing deeply nested PDF
objects, CVSS 7.5). `Cargo.lock` on `main` pins `lopdf` 0.41.0, which is
within the affected range; the advisory is fixed in `lopdf` >= 0.42.0.

### Affected dependency chain

- `lopdf 0.41.0` (locked in `Cargo.lock`)
- pulled in by `pdf-inspector 0.1.7`, which is used by the anydoc
document-reading path (`pdf-inspector` declares `lopdf` and resolves
the vulnerable version)

### Reproduction (from a clean checkout)

1. `git clone` the repository and check out `main`.
2. Run `cargo audit` (or `cargo deny check advisories`).
3. Observed: `error[vulnerability]: Stack overflow in lopdf via deeply
nested PDF objects / lopdf 0.41.0` — exit code 1.

### Suggested fix

Upgrade `pdf-inspector` from 0.1.7 to 0.1.8, which raises its `lopdf`
requirement to >= 0.42.0 and resolves the advisory. No application
source changes are needed. I have a patch ready and will open a PR
referencing this issue.

Bắt đầu với Cargo.lock và tái hiện advisory bằng cargo audit hoặc cargo deny check advisories. Kiểm tra dependency pdf-inspector trong đường dẫn đọc tài liệu của anydoc, sau đó xác minh rằng phiên bản lopdf được khóa không còn bị ảnh hưởng và kiểm tra advisory đã thành công.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.