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The plan and status count missing, outdated (another format) and
ready previews; the missing job never writes beside an outdated one.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/admin.py

@@ -18,6 +18,7 @@ from pathlib import Path
 from aiohttp import web
 
 from .prompts import thumbnail_url_for
+from . import thumbnail_settings
 from .jobs import (
     DEFAULT_SCAN_SCOPE,
     SCAN_GROUP_NAMES,
@@ -146,39 +147,48 @@ def _physical_file_key(path, stat):
     return ("path", os.path.normcase(os.path.realpath(str(path))))
 
 
-def _thumbnail_rel_path(rel_path, thumbnail_ext):
-    """Relative path of the thumbnail the gallery would generate for ``rel_path``."""
-    return f"thumbnails/{rel_path.with_suffix('').as_posix()}_thumb{thumbnail_ext}"
-
-
-def _media_thumbnail_rel_path(rel_path):
-    """The thumbnail of a media file: a video's preview is a ``.jpg``."""
-    extension = rel_path.suffix.lower()
-    return _thumbnail_rel_path(
-        rel_path, ".jpg" if extension in VIDEO_EXTENSIONS else rel_path.suffix
-    )
+def _other_sources(output_path, rel_path):
+    """Whether another media file shares *rel_path*'s stem in its folder."""
+    folder = Path(output_path) / Path(rel_path).parent
+    stem = Path(rel_path).stem
+    own = Path(rel_path).suffix.lower()
+    for extension in IMAGE_EXTENSIONS + VIDEO_EXTENSIONS:
+        if extension == own:
+            continue
+        for name in (stem + extension, stem + extension.upper()):
+            if (folder / name).is_file():
+                return True
+    return False
 
 
 def _remove_thumbnail(output_path, rel_path, logger=None):
-    """Remove the thumbnail of ``rel_path`` under ``output_path``, if any.
+    """Remove the previews of ``rel_path`` under ``output_path``, if any.
 
-    Shared by the legacy duplicate delete and the reviewed removal job. A
-    thumbnail that is a link, or one that cannot be removed, is left alone
-    (a stale preview never fails a removal). Returns whether one was removed.
+    Shared by the legacy duplicate delete and the reviewed removal job.
+    Every preview the file owns goes, in any format (#317); a preview name
+    another same-stem file still shares (``x_thumb.webp`` of ``x.png`` and
+    ``x.mp4``, ruling R2) stays for it. A preview that is a link, or one that
+    cannot be removed, is left alone (a stale preview never fails a removal).
+    Returns whether one was removed.
     """
     output_path = Path(output_path)
-    thumbnail_path = output_path / _media_thumbnail_rel_path(Path(rel_path))
-    try:
-        if thumbnail_path.is_symlink() or not thumbnail_path.is_file():
-            return False
-        os.remove(thumbnail_path)
-    except OSError as e:
+    rel_path = Path(rel_path)
+    shared = _other_sources(output_path, rel_path)
+    removed = False
+    for thumbnail_path in thumbnail_settings.owned_thumbnails(output_path, rel_path):
+        extension = thumbnail_path.suffix.lower()
+        if shared and extension in thumbnail_settings.WRITTEN_EXTENSIONS:
+            continue
+        try:
+            os.remove(thumbnail_path)
+        except OSError as e:
+            if logger is not None:
+                logger.warning(f"Could not delete thumbnail for {rel_path}: {e}")
+            continue
+        removed = True
         if logger is not None:
-            logger.warning(f"Could not delete thumbnail for {rel_path}: {e}")
-        return False
-    if logger is not None:
-        logger.debug(f"Deleted associated thumbnail: {thumbnail_path}")
-    return True
+            logger.debug(f"Deleted associated thumbnail: {thumbnail_path}")
+    return removed
 
 
 async def _read_json(request):
@@ -845,8 +855,13 @@ class AdminRoutesMixin:
         extension = media_path.suffix.lower()
         is_video = extension in VIDEO_EXTENSIONS
 
-        thumb_rel = _thumbnail_rel_path(rel_path, ".jpg" if is_video else extension)
-        thumbnail_url = thumbnail_url_for(thumb_rel, output_path / thumb_rel)
+        existing = thumbnail_settings.existing_thumbnail(output_path, rel_path)
+        thumbnail_url = None
+        if existing is not None:
+            thumb_rel = (
+                thumbnail_settings.thumbnail_stem_rel(rel_path) + existing.suffix
+            )
+            thumbnail_url = thumbnail_url_for(thumb_rel, existing)
 
         return {
             "id": str(hash(str(media_path))),