🐛 Bug: cleanup() in finally block creates BackgroundTask but never executes it — temp files leak

## Description

In `main.py`, the `cleanup(input_path)` call inside the `finally` block does **not** actually delete the file. It creates a `BackgroundTask` object that is immediately discarded.

## File & Line

`fastapi-docx/main.py`, lines 52, 55 (and same pattern in all other endpoints):

```python
finally:
cleanup(input_path) # ← Returns a BackgroundTask, doesn't execute!

return FileResponse(
path=output_path,
...
background=cleanup(output_path), # ← This one works because FileResponse runs it
)
```

## Why It Matters

Every uploaded file stays on disk forever in the temp directory (`TEMP_DIR`). Over time this fills up disk space. Only `output_path` gets cleaned (via FileResponse background task), but `input_path` never does.

## Suggested Fix

Either call the task directly:
```python
finally:
Path(input_path).unlink(missing_ok=True)
```

Or combine both paths into one background task:
```python
return FileResponse(
path=output_path,
...
background=cleanup(input_path, output_path),
)
```
and remove the `finally` block (but handle the error case separately to clean up input on failure).