Robust input validation and error handling for job initialization and preflight checks in an AI presenter video creation workflow.

## Summary

`scripts/init_job.py` crashes with `NotADirectoryError` when `--job-dir` points at an existing file. `scripts/preflight.py` crashes with `TypeError` when `input.supporting_media` is `null` in the job manifest.

Both are triggered by valid edge-case inputs and produce unhandled exceptions instead of the controlled error messages the scripts otherwise provide.

## Reproduction

### Bug 1: `--job-dir` pointing at a file

```bash
touch /tmp/test-file
python3 scripts/init_job.py --job-dir /tmp/test-file --presenter-image README.md --topic test
```

**Expected:** Controlled error (e.g. `"job directory path exists and is not a directory"`).

**Actual:**
```
NotADirectoryError: [Errno 20] Not a directory: /tmp/test-file
```

**Root cause:** `main()` calls `job_dir.iterdir()` before checking `job_dir.is_dir()`.

### Bug 2: `supporting_media: null` in job manifest

Create a `job.json` where `input.supporting_media` is `null` (no key or explicit `null`), then run:

```bash
python3 scripts/preflight.py job.json
```

**Expected:** `ok: true`, empty supporting media list.

**Actual:**
```
TypeError: NoneType object is not iterable
```

**Root cause:** `input_data.get("supporting_media", [])` returns `None` when the key exists with value `null`; the default is only used when the key is absent.

## Proposed fix

1. **`scripts/init_job.py`**: Add `is_dir()` guard before `iterdir()`.
2. **`scripts/preflight.py`**: Normalize `supporting_media` (handle `None`, `""`, scalar string, no...