`list issues` is currently unusable: `category` required by API, no flag exposed

## What

`fossapi list issues` today always fails:

```
$ fossapi list issues --json --count 1
Error: FOSSA API error: Validation error: Invalid option: expected one of
"licensing"|"vulnerability"|"quality" at "category"
```

The underlying `/api/v2/issues` endpoint requires a `category` query param,
but the CLI neither passes a default nor exposes a flag.

## Proposed fix

Add a `--category` flag (required, or defaulting to `licensing`), and while
you're in there, add filters the v2 endpoint supports:

```
fossapi list issues --category licensing|vulnerability|quality
[--status active|ignored|all]
[--page N] [--count N]
[--json]
```