test(e2e): drive write-to-file per format in the real UI; fix sidecar perms

Per the owner's testing bar: a Playwright spec registers an admin,
seeds one fixture per write-capable type (EPUB, PDF, CBZ, MP3, FLAC,
M4A, M4B — generated at runtime, no binary media in git), drives the
REAL metadata-editor flow (Edit → change title → in-app dialog →
Save & write) for each, and then reads every artifact back
INDEPENDENTLY of the app — EPUB/CBZ/sidecar via python zip+XML parse,
audio via ffprobe — asserting the written title landed in the FILE and
the old one is gone.

Writing the e2e exposed a real bug: atomicWrite left server-written
sidecars at 0600 root (os.CreateTemp's default), so host-side tools
sharing the volume — Calibre reading metadata.opf — were locked out.
The temp file is now chmod'd 0644 before the rename, matching the
scanner's cover-file convention; the e2e's host-side reads prove it.

internal/services/metadata_writer.go

@@ -417,6 +417,14 @@ func atomicWrite(path string, data []byte) error {
 		return err
 	}
 	tmpPath := tmp.Name()
+	// 0644: sidecars/rewritten files must be readable by host-side
+	// tools sharing the volume (Calibre reads metadata.opf); a 0600
+	// temp default would lock everyone but the server out.
+	if err := tmp.Chmod(0o644); err != nil {
+		tmp.Close()
+		os.Remove(tmpPath)
+		return err
+	}
 	if _, err := tmp.Write(data); err != nil {
 		tmp.Close()
 		os.Remove(tmpPath)