docs(monitoring): record legacy poller decommission (slice 3)

Update docs to reflect that Manage no longer scrapes its own system
metrics (slices 1-2). AGENTS.md, REQUIREMENTS.md (decision log +
observability section), monitoring-logging-design.md, MIGRATION_PLAN.md.

Gate: docs only; backend pytest (173) + frontend build/lint/test (22/63)
remain green from slices 1-2.

AGENTS.md

@@ -1,12 +1,14 @@
 # AGENTS.md
 
 ## Layout
+
 - Current app is `backend/` (FastAPI) plus `frontend/` (Vite React); ignore Streamlit-era commands in `CONTRIBUTING.md`.
 - Backend entrypoint: `backend/src/media_library_viewer_api/main.py` (`media_library_viewer_api.main:app`).
 - Frontend entrypoint: `frontend/src/main.tsx`.
 - Backend uses a `src/` layout; tests live in `backend/tests/`.
 
 ## Commands
+
 - Backend setup: `cd backend && python -m venv .venv && source .venv/bin/activate && pip install -e '.[dev]'`
 - Backend run: `uvicorn media_library_viewer_api.main:app --reload --port 8000`; if not installed, use `PYTHONPATH=src uvicorn media_library_viewer_api.main:app --reload --port 8000`.
 - Backend tests: run `pytest` from `backend/`; focused checks can use `pytest tests/test_api.py` or `pytest -k <expr>`; if the package is not installed, use `PYTHONPATH=src pytest`.
@@ -17,12 +19,13 @@
 - Production stack: `docker compose up --build`
 
 ## Repo-Specific Gotchas
+
 - Root compose files rely on environment-variable interpolation, not `env_file`; export required values before running them.
 - Production compose needs the host/cert and OIDC variables from `docker-compose.yml` (`BACKEND_APP_HOST`, `FRONTEND_APP_HOST`, `CERT_RESOLVER`, and the frontend OIDC vars).
 - Dev compose runs with auth off and does not need SSH key material unless you add remote SSH machines.
 - `backend_cache` persists the media index and the managed `known_hosts` file.
 - SSH host-key checking is strict, but the first successful connect records the host key into backend-managed `known_hosts`.
-- Backend startup validates auth settings, rebuilds managed `known_hosts`, and starts the mail queue and monitoring poller.
+- Backend startup validates auth settings, rebuilds managed `known_hosts`, and starts the mail queue and backup alert poller.
 - Machine-level settings now own Jellyfin/Jellyseerr/SSH config; the backend seeds a local machine automatically.
 - Remote job templates live in `backend/src/media_library_viewer_api/jobs.py`; keep shell quoting intact.
 - Backend Ruff config is in `backend/pyproject.toml` and uses line length 120 with Python 3.11.