feat(thumbnails): every preview writer and lookup follows the thumbnail settings

Automatic previews (capture and first view), the thumbnail job, the
legacy gallery routes and the history route write <stem>_thumb.webp
(or .jxl/.jpg) with the configured quality and size limit, for images
and videos alike: video frames, ffmpeg's included (a PNG on stdout,
never upscaled), and the placeholder are encoded like images.

Listings, duplicates, enrichment, the autotag scan and the history
route show any preview the file owns, the current format first, so
older _thumb.png previews keep working until converted. A request for
an older name answers the current preview. Previews are served with
an explicit image/webp or image/jxl type and the same cache headers.

The plan and status count missing, outdated (another format) and
ready previews; the missing job never writes beside an outdated one.
Removing a file removes its previews in every format; the clear
removes .webp and .jxl too. Encoder processes are killed at shutdown.

A preview is only a placeholder: inspection refuses paths under
thumbnails/ and always reads the original, and the inspector cover
accepts the listing's preview in any format the file owns.

py/api/thumbnail_encode.py

@@ -34,10 +34,13 @@ import os
 import shutil
 import subprocess  # nosec B404 - fixed argv lists, never a shell
 import threading
+from dataclasses import replace
 from pathlib import Path
 
 from PIL import Image
 
+from . import thumbnail_settings
+
 try:
     from ...utils.logging_config import get_logger
 except ImportError:  # tests import the package top-level
@@ -383,6 +386,17 @@ def _jxl_fallback():
         )
 
 
+def writer_settings(settings=None):
+    """The settings a preview is written with now: the configured ones, but
+    WebP while JPEG XL has no encoder (logged once). Its extension names the
+    file, so "outdated" means a preview this server would not write now."""
+    settings = settings or thumbnail_settings.current()
+    if settings.format == "jxl" and detect_jxl_encoder() is None:
+        _jxl_fallback()
+        return replace(settings, format="webp")
+    return settings
+
+
 def write_thumbnail(frame, dst, settings, *, guard=None, track=None):
     """Encode *frame* into *dst* in ``settings``' format; returns the path
     written (``.webp`` when JPEG XL fell back). Atomic and verified; raises