enh(relay): send the relay API a path relative to the camera dir

[JamBalaya56562]

What

Make the relay-event API receive a path relative to the camera dir instead of the absolute %f path motion passes. relayevent.sh strips the camera dir before posting, so the validation-oriented path slicing in relay_event.py and mediafiles.make_movie_preview is no longer needed.

Implements the idea @MichaIng raised in #3374 ("I would actually prefer if the API itself would take a relative path ... split off the camera dir in relayevent.sh").

How

• scripts/relayevent.sh — takes an optional camera_dir arg and strips it from the filename (${filename#"$camera_dir"/}). Guarded against an empty/non-matching value, so a stale absolute path is left untouched and still validated server-side.
• config.py — the generated on_movie_end / on_picture_save commands now pass the camera's target_dir (single-quoted) as that argument. target_dir is already known where the command is built; motion has no conversion specifier for it.
• handlers/relay_event.py — the incoming filename is now relative. remove_prefix still tolerates a legacy absolute path (un-regenerated motion config), then it's validated directly. The absolute path is reconstructed only for the upload call, so the upload subsystem is unchanged.
• mediafiles.make_movie_preview — now takes a relative path, validates it, and reconstructs the absolute path internally; get_media_preview passes the relative path it already had (dropping a redundant os.path.join).

Scope / not included

The upload services (UploadService.upload_file, incl. the S3 override) still receive an absolute path and keep deriving the cloud-relative name themselves — that derivation is cloud-path naming tied to @upload_subfolders, a separate concern from the validation slicing this PR removes. Relativizing it (and the broader "every API endpoint takes relative paths") would be a follow-up. Builds on the now-merged #3374 (utils.remove_prefix).

Testing

• Full suite green in a Linux container: 128 passed.
• tests/test_mediafiles.py: the make_movie_preview traversal/escape validation tests now pass relative malicious paths (the form the API actually receives); added a test asserting the absolute path is reconstructed (os.path.join(target_dir, rel_path)) for ffmpeg.
• shellcheck -o all passes on relayevent.sh; verified the prefix-strip with a shell harness (matching prefix → relative, non-matching → unchanged, spaces handled, empty camera_dir → no bare-slash strip).
• ruff check / ruff format --check pass.
• Note: an end-to-end run (real motion + camera) wasn't possible in my environment.