Prune stems whose source song was removed or replaced

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

server/README.md

@@ -130,6 +130,9 @@ the app would briefly see no stems.
 | `DEVICE`     | `cuda`           | `cuda`, `cuda:N` or `cpu`                            |
 | `RESCAN_MIN` | `30`             | minutes between scans once caught up                 |
 | `AUTO_SCAN`  | `0`              | `1` = scan the library every `RESCAN_MIN`; `0` = only do Stem Now requests |
+| `PRUNE`      | `1`              | Remove stems whose song is gone from the library, at start and every `RESCAN_MIN` |
+| `PRUNE_CHANGED` | `0` | Also remove stems of songs replaced in place (size or mtime changed) |
+| `PRUNE_MAX` | `0.2` | Skip a prune that would remove more than this share of the stems. `server/prune.py` is the manual dry run, `--apply` to remove |
 | `FP16`       | `1`              | half precision on the GPU (`0` for fp32)             |
 | `OVERLAP`, `SEGMENT`, `SHIFTS` | `0.25`, `0`, `0` | as in the desktop app's settings     |