Add --rights <text> flag for stamping dc:rights (closes #21)

[roelvangils]

Summary

Tiny feature implementing #21. `dpub convert --rights ""` stamps a free-text `dc:rights` element into the EPUB's OPF metadata.

Why

Producers redistributing accessibility-exception copies routinely need to assert the rights frame they're operating under — copyright statement, Marrakesh Treaty, Belgian / EU exception, etc. DAISY 2.02 NCCs sometimes carry that in `dc:rights` and sometimes don't (the reference Belgian title in this project's memory does not). Before this PR, dpub had no way to add it.

Behaviour

• Source-NCC fallback: `build_package_metadata` defaults `rights` from the source NCC's `other.get("dc:rights")` so books that already carry rights info don't lose it on conversion.
• CLI override: `--rights ""` wins over the source.
• Omitted flag + missing source = no `dc:rights` element emitted (current behaviour preserved).

Implementation

• `PackageMetadata.rights: Option` in `epub3-writer/src/model.rs`. Surfaced as `dc:rights` in `writers.rs::write_opf_metadata`.
• `ConvertOptions.rights: Option` in `dpub-convert/src/lib.rs`. Override applied after the initial `convert(book)` call.
• CLI flag in `dpub-cli/src/main.rs`.

Out of scope

• `--rights-from-file ` for longer text. Trivial follow-up if needed.
• Structured rights statements (CC URIs, Marrakesh-statement libraries). Plain text is enough for v1.

Test plan

• `cargo test --workspace` green.
• `cargo clippy -p epub3-writer -p dpub-convert -p dpub-cli --all-targets -- -D warnings` clean.