Implemented shared S3 audio caching for prepare and restore --include-audio

docs/config.md

@@ -165,6 +165,10 @@ spool:
   root: /var/spool/narratio
   delete_audio_after_archive: true
 
+cache:
+  root: /var/cache/narratio
+  s3_audio: true
+
 archive:
   enabled: true
   upload_run: true
@@ -261,6 +265,8 @@ Operational notes:
 | `pipeline.storage.s3.secret_access_key_env` | string | No | `OBJECT_STORAGE_KEY` |
 | `pipeline.spool.root` | string | No | `/var/spool/narratio` |
 | `pipeline.spool.delete_audio_after_archive` | bool | No | `false` |
+| `pipeline.cache.root` | string | No | `/var/cache/narratio` |
+| `pipeline.cache.s3_audio` | bool | No | `true` |
 | `pipeline.archive.enabled` | bool | No | `true` |
 | `pipeline.archive.upload_run` | bool | No | `true` |
 | `pipeline.archive.promote_artifacts[]` | list | No | trimmed transcript rule |
@@ -401,6 +407,7 @@ Restore-related implications:
 
 - restore remote identity requires archive S3 identity to resolve (`pipeline.storage.s3.bucket` and session prefix derivation inputs).
 - restore scope considers committed current state and durable paths (`manifest.json`, `transcripts/**`, `artifacts/**`, `previous/**`, optional `audio/**`).
+- S3 audio downloads use `pipeline.spool.root` for active downloads and `pipeline.cache.root` for reusable cached audio when `pipeline.cache.s3_audio` is true.
 
 ## 7. Full campaign reference