Fix specialty mismatch

[zmek]

Summary

• Separate services and subgroups into distinct keys ("services" / "subgroups") in the transfer model's internal dict, so subgroup names (e.g. paediatric, adult_male_young) no longer appear alongside real services in error messages
• Return 0.0 with a warning when get_transfer_prob is called with a service not in the training set, instead of crashing with a ValueError that dumps hundreds of service names
• Shorten warn_specialty_mismatch messages to report counts only, without listing every mismatched specialty
• Add prettier formatting step to tox -e docs (pinned to @3.1.0) so generated notebook markdown matches the pre-commit hook and doesn't create spurious diffs
• Fix task issue template formatting

Test plan

• Existing tests pass (221 tests, all green)
• New test: unknown service returns 0.0 with warning
• New test: internal dict correctly separates services from subgroups
• uv run tox -e docs followed by pre-commit produces no changes

[zmek]

Closes #147