Sync cached files and refuse ones left empty or partial (closes #252)

Source and variant files are synced to disk before the rename into
place, and their directory after. Lookup and LookupSource compare the
file's size with the size its row records: a file that is empty or of
another size is removed with its rows and a warning naming it, and the
request converts the image or fetches the source again. A variant whose
row cannot be read is judged by its file alone. Storing a source over a
file of another size replaces it. A source that fails to decode is not
removed for that alone.

#252

Model: opus-5-5

TODO.md

@@ -55,6 +55,15 @@ P2: security: per-origin rate limiting
   answered 500. Once the limit is used up, the fetcher now reads one more byte:
   if the body has ended it is accepted, and if there is more it is refused with
   `ErrResponseTooLarge`, as before.
+- 2026-10-15 a cached file left empty or partial is no longer served, or fetched
+  again on every request (closes #252). Source and transformed image files are
+  synced to disk before they are renamed into place, and their directory after.
+  A request that finds a cached source or transformed image whose file is empty,
+  or not the size the database records for it, removes it with a log line naming
+  it, and fetches the source or converts the image again. Storing a source over
+  a file of another size replaces that file. A source that fails to decode is
+  not removed for that alone: a damaged upstream image would then be fetched on
+  every request.
 - 2026-10-13 `fit=outside` no longer makes an output above 8192 on a side
   (closes #257): it scales the image to cover the requested size, and the side
   scaled past that size was held to no maximum, so `1x8192?fit=outside` of a