Split deploy into independent poller and web stacks

Decouple the privileged hardware poller from web iterations so a web
redeploy can never recreate/restart the poller (the backplane-touching
container). Two images from one Dockerfile via build targets:

- jbod-poller: privileged producer, ships smartmontools/sg3-utils/ledmon,
  Redis-only deps (~197MB)
- jbod-web: unprivileged read-only consumer (REST/UI/MQTT), no hardware
  tools, no /dev (~147MB)

Two compose projects sharing Redis over host networking:
- compose.infra.yml (jbod-infra): poller + redis — stable substrate
- compose.web.yml   (jbod-web):   app — 'up -d --build' touches only app

build.sh builds/pushes both images; split requirements-{poller,web}.txt;
dropped the combined docker-compose.yml; .dockerignore excludes tmp/ (keeps
the venv and the mqtt.env creds out of the build context).

README.md

@@ -45,12 +45,21 @@ apt install smartmontools sg3-utils ledmon   # Debian/Ubuntu
 
 ## Run (docker compose)
 
+Two **independent** stacks so web iterations never recreate the privileged
+poller (which touches the SAS bus). They're built as two images —
+`jbod-poller` (privileged, has the SAS/SMART tooling) and `jbod-web` (slim,
+unprivileged) — and share Redis over host networking.
+
 ```bash
-docker compose up -d --build
+# 1. Infra: poller + redis — the stable substrate. Deploy once; touch deliberately.
+docker compose -f compose.infra.yml up -d
+
+# 2. Web: REST/UI/MQTT — iterate freely; this only ever recreates `app`.
+docker compose -f compose.web.yml up -d --build
 ```
 
-Brings up `poller` (privileged), `app` (port 8000), and `redis`. To run the two
-processes by hand instead:
+Pin both images to a known-good SHA in deploy (`./build.sh` tags `:<sha>` and
+`:latest` for both). To run the two processes by hand instead:
 
 ```bash
 REDIS_HOST=localhost sudo -E python poller.py            # producer (needs root)