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

A crash could leave a cached source or transformed image empty or cut
short, and pixa then served it, or fetched it again forever. Source and
transformed image files, and the .meta file beside each transformed
image, are now written under a temporary name, synced, renamed into
place and their directory synced; a failed directory sync removes the
renamed file. The reconciliation pass compares each cached file's size
with the size its row records and removes a file that is empty or of
another size, with its rows and a warning. Requests compare no sizes.

Not verified by a test: the syncs, and the removal after a failed sync.

Model: opus-5-5

internal/imgcache/cache.go

@@ -660,10 +660,10 @@ func (c *Cache) readHitAndMissCounts(ctx context.Context, stats *CacheStats) err
 // hold: it reads the content type from the variant's .meta file and
 // keeps it in metaCache, unless a StoreVariant has put one there
 // meanwhile, as the store's is newer. A read that finds no .meta file,
-// as one can between a store's writing of the variant file and of its
-// .meta file, serves application/octet-stream and keeps nothing, so
-// metaCache only ever holds a type read from a .meta file or passed to
-// StoreVariant.
+// as one can when the store's .meta write failed or an eviction removed
+// the file after the read opened the variant, serves
+// application/octet-stream and keeps nothing, so metaCache only ever
+// holds a type read from a .meta file or passed to StoreVariant.
 func (c *Cache) loadVariantWithMeta(
 	cacheKey VariantKey,
 ) (io.ReadCloser, int64, string, error) {