web: audiobook player — stitched-file playback with chapter seek

Adds GET /stories/{id}/listen: one <audio> element over a story's
stitched audiobook file plus a clickable chapter list. Clicking a
chapter seeks; the chapter under the playhead highlights as it
plays. Chapter offsets are summed from each chapter's latest
succeeded narration_run duration — the same order the file was
stitched. One small inline script, the web UI's first JS.

New stories.audiobook_path column (migration 0009) holds the
served path; the story page shows a "listen" action when set.

migrations/0009_story_audiobook.sql

@@ -0,0 +1,7 @@
+-- A story can have a single stitched audiobook — all its chapter
+-- renders concatenated into one chaptered file (see the m4b built
+-- from per-chapter narration_runs). audiobook_path is the path the
+-- web server serves it from (e.g. /audio/The-Coast-Down.m4b); NULL
+-- means no audiobook has been stitched yet.
+ALTER TABLE stories
+    ADD COLUMN IF NOT EXISTS audiobook_path text;