fix: handle subprocess output decoding

[pixelcola]

Summary

• decode captured NSSM subprocess output with explicit UTF-8 and replacement handling
• add a regression test for invalid output bytes
• document the subprocess output decoding contract in backend specs

Verification

• UV_CACHE_DIR=/tmp/uv-cache uv run ruff check src
• UV_CACHE_DIR=/tmp/uv-cache uv run ty check src
• UV_CACHE_DIR=/tmp/uv-cache uv run pytest