fix: get_cats reports "*" default-category sentinel

[thedancingdeveloper]

Summary

Fixes #73: mode=get_cats should report the default category as the literal string "*", not "Default".

Verified against sabnzbd/sabnzbd@5.1.x:

# sabnzbd/api.py:792-793
def _api_get_cats(name, kwargs):
    return report(keyword="categories", data=list_cats(False))

# sabnzbd/api.py:2090-2098
def list_cats(default: bool = True) -> list[str]:
    lst = [cat["name"] for cat in config.get_ordered_categories()]
    if default:
        lst.remove("*")
        lst.insert(0, "Default")
    return lst

_api_get_cats explicitly calls list_cats(False), so the real response is {"categories": ["*", "movies", "tv", ...]} — the "*"→"Default" substitution only happens for the internal config UI.

Impact

Likely related to #65: a client that specifically recognizes "*" as the default-category sentinel (rather than treating the array as opaque display names) wouldn't find it in RustNZB's response.

Fix

handle_get_cats now emits "*" for the default category. Since RustNZB's own category model still names it "Default" internally, added sab_resolve_category() to translate "*" back to "Default" wherever a client-supplied cat value is applied (addfile, addurl, change_cat), keeping both directions of the boundary translation consistent.

Test plan

• cargo test -p nzb-web — added get_cats_reports_default_category_as_sabnzbd_sentinel and change_cat_accepts_sabnzbd_default_sentinel.
• cargo clippy -p nzb-web --all-targets — clean.
• Full nzb-web test suite passes.

🤖 Generated with Claude Code

[thedancingdeveloper]

Superseded — this change landed via #84, which stacked all of #71-#77's fixes together and was squash-merged to save on redundant CI runs. Closing without merging to avoid re-applying the same diff.