feat(content): serve the mounted content folder under /content (spec 003)

Audio with range requests, library.json, Homer books; LRC parser. Content
stays out of git and the image.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DECISIONS.md

@@ -66,3 +66,11 @@ persistent side panel. No new dependency.
 ## 2026-10-30 — three.js allowed in mockup 16
 
 **Decision:** mockup 16 (Voyage, readable) may use three.js; it iterates on 15. No new dependency.
+
+## 2026-10-30 — Real content is mounted, never committed (spec 003)
+
+**Decision:** the real audio, timed lyrics, cover art and Homer texts stay in a folder outside the repo
+(`~/epic-the-musical-content` on the dev host), served under `/content/` from `CONTENT_DIR` and mounted
+read-only into the container. **Why:** the audio, lyrics and art are Jorge Rivera-Herrans' copyrighted
+work; committing them would push them to Gitea and bake 1.5 GB into the image. The user chose this
+("Mount it, never commit"). Range requests are handled in `src/content/routes.ts` so audio can seek.