Collections with exclamation mark - broken image in saved posters

### Description

Collection names containing an exclamation mark (`!`) can result in broken images under:

`Posters -> Collection Posters -> Saved Posters`

In my case the collection names came from Maintainerr 3.7.0 and included `!`:

- `Movies leaving stuartmedia Soon!`
- `Shows Leaving Soon!`

Agregarr generated/saved poster files using filenames derived from those collection names:

```text
Movies_leaving_stuartmedia_Soon!_(Movies).jpg
Shows_Leaving_Soon!_(TV_Shows).jpg
````

The files existed on disk and were readable inside the container, but the UI showed broken images and Agregarr logged `Poster file not found`.

This appears to be caused by a mismatch in filename handling:

* `sanitizeFilename()` allows `!` to remain in generated filenames.
* `isValidFilename()` later rejects filenames containing `!`.
* `posterExists()` then returns `false`.
* The UI route reports `Poster file not found`, even though the file exists.

So this is not a filesystem, Docker, mount, or permission issue. It appears to be Agregarr creating/storing a filename that its own poster-serving validation later rejects.

## Expected Behavior

Agregarr should either:

* sanitize `!` out of generated poster filenames before saving them, or
* allow `!` consistently in `isValidFilename()` if that character is intended to be supported.

The app should not create/store poster filenames that it later refuses to serve.

## Actual Behavior

Agregarr stores/references poster filenames containing `!`, but later fails to serve them.

Example log:

```text
Failed to load poster file: Poster file not found: Shows_Leaving_Soon!_(TV_Shows).jpg
Poster file not found or failed to load {"filename":"Shows_Leaving_Soon!_(TV_Shows).jpg","error":"Failed to load poster file"}

Failed to load poster file: Poster file not found: Movies_leaving_stuartmedia_Soon!_(Movies).jpg
Poster file not found or failed to load {"filename":"Movies_leaving_stuartmedia_Soon!_(Movies).jpg","error":"Failed to load poster file"}
```

But the files existed inside the container:

```text
/app/config/posters/Movies_leaving_stuartmedia_Soon!_(Movies).jpg
/app/config/posters/Shows_Leaving_Soon!_(TV_Shows).jpg
```

Container-side validation:

```text
uid=1000(node) gid=10000 groups=10000

/app/config/posters:
-rw-r--r-- 1 node 10000 210.7K Movies_leaving_stuartmedia_Soon!_(Movies).jpg
-rw-r--r-- 1 node 10000 185.4K Shows_Leaving_Soon!_(TV_Shows).jpg

shows poster visible
movies poster visible
```

The references also existed in `settings.json`:

```text
"customPoster": "Movies_leaving_stuartmedia_Soon!_(Movies).jpg"
"customPoster": "Shows_Leaving_Soon!_(TV_Shows).jpg"
```

### Version

2.4.2

### Steps to Reproduce

1. Have Maintainerr create or manage a collection with `!` in the collection name.

* Example: `Shows Leaving Soon!`
* Example: `Movies leaving stuartmedia Soon!`
2. In Agregarr, run/sync the Plex collections so the renamed/managed collections are discovered.
3. Run poster-related jobs / poster overlay application.
4. Go to:

* `Posters -> Collection Posters -> Saved Posters`
5. Observe broken poster images.
6. Check Agregarr logs.

I do not know if this only occurs with Maintainerr-created collections, but based on the source code it looks like any flow that produces a poster filename containing `!` could trigger the same problem.

### List Links

_No response_

### Screenshots

I do not have screenshots available, but the visible symptom was broken images under:

`Posters -> Collection Posters -> Saved Posters`

### Logs

```shell
2026-04-25T16:38:19.478Z [error]: Failed to load poster file: Poster file not found: Shows_Leaving_Soon!_(TV_Shows).jpg {"stack":"Error: Poster file not found: Shows_Leaving_Soon!_(TV_Shows).jpg\n at loadPosterFile (/app/dist/lib/posterFileManager.js:76:19)\n at /app/dist/routes/posters.js:827:79"}

2026-04-25T16:38:19.479Z [debug]: Poster file not found or failed to load {"filename":"Shows_Leaving_Soon!_(TV_Shows).jpg","error":"Failed to load poster file"}

2026-04-25T16:38:19.481Z [error]: Failed to load poster file: Poster file not found: Movies_leaving_stuartmedia_Soon!_(Movies).jpg {"stack":"Error: Poster file not found: Movies_leaving_stuartmedia_Soon!_(Movies).jpg\n at loadPosterFile (/app/dist/lib/posterFileManager.js:76:19)\n at /app/dist/routes/posters.js:827:79"}

2026-04-25T16:38:19.481Z [debug]: Poster file not found or failed to load {"filename":"Movies_leaving_stuartmedia_Soon!_(Movies).jpg","error":"Failed to load poster file"}

Earlier related log entries:

2026-04-25T06:00:27.725Z [info][Poster Discovery]: Poster URL changed, deleting old file {"collectionTitle":"Movies leaving stuartmedia Soon!","oldPath":"Movies_leaving_stuartmedia_Soon!_(Movies).jpg"}

2026-04-25T06:00:27.734Z [info][Poster Discovery]: Poster URL changed, deleting old file {"collectionTitle":"Shows Leaving Soon!","oldPath":"Shows_Leaving_Soon!_(TV_Shows).jpg"}
```

### Additional Context

I worked around the issue by:

1. Removing `!` from the Maintainerr rule/collection name.
2. Running Maintainerr rules and allowing processing to complete.
3. Confirming the new collection name in Maintainerr.
4. Clicking `Handle collections` in Maintainerr and allowing that to complete.
5. Confirming the collections were renamed in Plex.
6. Running Agregarr `Plex collections sync`.
7. Confirming the rename under Agregarr `All Collections`.
8. Running poster overlay jobs.
9. Manually deleting the old `!` poster files from `/app/config/posters`.
10. Making a change to the default Agregarr poster template to force fresh poster generation.

After that, the errors stopped and Plex showed the updated collection posters.