fix: use runuser for privilege drop and capture container stderr in logs

The container still exited immediately after the su-based fix.  can
interfere with TTY/stdin handling for interactive shells. Switch to
, which is root-only, skips PAM, and preserves file descriptors so
bash stays interactive.

Also improve container failure diagnostics:
-  now combines stdout and stderr
- This helps surface the real reason when a container exits with code 0

Quality gates:
- pytest tests/unit: 219 passed
- ruff: clean on changed files
- mypy: clean on changed files

apps/api/src/services/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps/api/src/services
 
 ## role
-Marks the `services` directory as a Python package for organizing business logic and service-layer modules.
+Service layer package for the API application, intended to contain business logic implementations.
 ## parent
 index: apps/api/src/.pi-map.index.md
 map: apps/api/src/.pi-map.md