Keep cached sources and variants in one FileStore type (closes #318)

imgcache.FileStore is a directory of files named by a hex key, at
<ab>/<cd>/<key>. It writes a file under a temp name, syncs it and renames
it into place. It opens, reports the size of and deletes the file for a
key. The cache keeps its sources in one FileStore and its variants in
another, so ContentStorage and VariantStorage are gone. FileStore has no
Exists: the reconciliation pass checks a variant's file with Size, and the
tests look for the file on disk. Cache.Variant reads the content type from
the file's first bytes itself. storage.go is now filestore.go, and the
unused testdata/red.avif is gone.

Model: opus-5-5

TODO.md

@@ -30,6 +30,23 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 the cache keeps its source images in one `imgcache.FileStore` and
+  its variants in another (closes #318). A `FileStore` is a directory of files,
+  each named by a hex key, at `<ab>/<cd>/<key>`.
+    - `ContentStorage` and `VariantStorage` are gone.
+    - `FileStore` has these methods:
+        - `Write`, which writes a file under a temp name, syncs it and renames
+          it into place
+        - `Open`, which returns the file and its size
+        - `Size`
+        - `Delete`
+    - `FileStore` has no `Exists`. The reconciliation pass checks that a
+      variant's file is there with `Size`. The tests look for the file on disk.
+    - `Cache.Variant` reads the content type from the variant file's first
+      bytes, as `VariantStorage.LoadWithContentType` did. A cache hit still
+      makes no database call.
+    - `storage.go` is now `filestore.go`. `internal/imgcache/testdata/red.avif`,
+      which no test used, is gone.
 - 2026-10-26 `internal/imgcache` holds the cache only (closes #317). The new
   package `internal/imageproxy` fetches, transforms, caches and serves images.
   It imports `imgcache`, and `imgcache` does not import it.