show: keep a custom LICENSE in the Modelfile

[kartsan03]

/api/create takes a license for a derived model and stores it as the model's own license layer, and show returns it in license. The Modelfile from ollaya show --modelfile leaves it out, though:

POST /api/create {"model": "short", "from": "laya:en", "license": "MIT"}

# Modelfile generated by "ollaya show"
# To build a new Modelfile based on this, replace FROM with:
# FROM short:latest

FROM laya:en
DESCRIPTION laya:en for tests

So ollaya create -f on that Modelfile builds a model with the parent's license instead of MIT. This is the same round-trip loss that #57 fixed for DESCRIPTION and #55 fixed for CALIBRATION.

Change: in views::show, a derived model whose license layer differs from its parent's gets a LICENSE """ ... """ block. It's always a block: modelfile::parse reads a one-line LICENSE value as a path to a license file, so LICENSE MIT would look for a file named MIT. A license the model shares with its parent stays out of the Modelfile, the same digest rule #55 uses for calibration. The parent manifest is now read once for both checks.

Test: show_modelfile_keeps_a_custom_license in crates/ollaya-server/tests/http.rs creates two derived models, one with MIT and one with a multi-line license, and checks for the LICENSE block in each show. Then it creates a child of the second one and checks that the child's Modelfile has no LICENSE. It fails on main and passes with this change.

cargo fmt --all --check
cargo clippy --workspace --all-targets --locked -- -D warnings
cargo test --workspace --exclude ollaya --exclude ollaya-runner --locked   # http: 17 passed

I couldn't link ollaya and ollaya-runner on this machine: the prebuilt ONNX Runtime needs a newer glibc than it has. This change doesn't touch either crate, and clippy checked both.