Docs: production deployment recipe + Arch gotchas

- README: expand the production section with a full systemd recipe
  (useradd, chown, uv sync --project, install -d for ReadWritePaths,
  .env perms 640), a Caddy block, and backup note.
- AGENTS.md: add the Arch python-uv package-name note and the root-rooted
  venv caveat, plus a new "Deploy / production" section documenting the
  volundr unit, data location, .env perms, and forward-proxy expectations.

AGENTS.md

@@ -28,6 +28,12 @@ uv run ruff format app tests               # apply formatting
 There is **no `main.py` at the repo root** — the app is the `app` package,
 entry point `app.main:app`.
 
+**Arch Linux note:** `uv` ships as `python-uv` (not `uv` or `python3-uv`) so
+`sudo pacman -S --needed python-uv` is the install command. If you create the
+venv with `sudo`, the interpreter will provision under `/root/.local/share/uv`
+and a non-root service user can never execute it; build the venv as the
+service user (or against a system Python — see README "systemd recipe").
+
 ## Configuration
 
 All settings come from env vars (or a `.env` file) via `pydantic-settings`,
@@ -45,6 +51,30 @@ All settings come from env vars (or a `.env` file) via `pydantic-settings`,
   lead time, default 60), `BIKE_ADMIN_RENEWAL_MONTHS` (MOT/insurance validity,
   default 12).
 
+## Deploy / production
+
+`deploy/bike_admin.service` is the systemd unit for the production server
+(volundr). Key points:
+
+- Runs as the unprivileged `bike` system user (no home dir needed beyond the
+  uv-provisioned interpreter in `/home/bike`.local/share/uv/...).
+- App data lives next to the checkout: `/opt/bike_admin/data`. The app creates
+  `data/bike_admin.db` on startup, but systemd requires the `ReadWritePaths=`
+  target to exist before start — hence the one-time
+  `sudo install -d -o bike -g bike /opt/bike_admin/data` step (see README).
+- `.env` at `/opt/bike_admin/.env`, `chown bike:bike` and `chmod 640`.
+  (systemd reads it at ExecStart.)
+- Hardening kept: `NoNewPrivileges`, `PrivateTmp`, `ProtectSystem=full`,
+  `ProtectHome=true`. Do not remove `ProtectHome` unless the interpreter path
+  moves back to `/home`; `StateDirectory=` is the cleaner alternative for
+  data, but we chose `ReadWritePaths=` to keep data next to the app.
+
+Caddy (hermodr, on a separate host) reverse-proxies
+`volundr.local.jakes-asgard.uk` to the app's IP:8000. The app trusts
+`X-Forwarded-For` (BIKE_ADMIN_TRUST_PROXY defaults true) and uvicorn's
+`--forwarded-allow-ips` whitelist in the unit governs which proxies it'll
+accept.
+
 ## Architecture / layout
 
 - `app/config.py`  – `Settings` (pydantic-settings) + `get_settings()` (cached).