fleet-manager: fix nonroot data-volume ownership + document broker wiring

The distroless nonroot runtime (uid 65532) couldn't write /data/inventory.json
because a named volume is created root-owned — approve failed with permission
denied on the inventory birth. Pre-create /data owned by nonroot in the image so
a fresh volume inherits it via Docker copy-up; an already-created root-owned
volume still needs a one-time chown (documented).

DEPLOY.md:
- 2b now covers BOTH broker deployments — the main serving stack
  (enrol-pki/broker, ENROL_ADMIN_INSECURE=true by default → must flip to false)
  and the bench stack — with the mint-once-into-broker-dir, copy-CA-to-manager
  flow and an mTLS verify curl.
- Troubleshooting section: the data-volume permission-denied fix, the
  "authorise plane not configured" (recreate + single-instance) fix, and the
  "wrong version number" (broker plaintext) fix.

serving/fleet-manager/DEPLOY.md

@@ -57,12 +57,40 @@ AGENT_SANS=diskless-server.bitasylum.org,10.0.0.1 \
 
 The manager is an mTLS **client** of the enrol-broker's admin plane. Mint its
 client cert from the broker's **dedicated admin CA** (NOT the fleet CA — reusing
-the fleet CA would let any enrolled host authenticate to the admin plane):
+the fleet CA would let any enrolled host authenticate to the admin plane). Mint
+the whole set **once** into the broker's own PKI dir (so the broker gets its
+`server.crt`/`server.key` too), then copy the CA + client cert to the manager so
+**both sides share one admin CA**.
 
+The broker's PKI dir + its "serve mTLS" switch differ by which stack runs it:
+
+- **Main serving stack** (`serving/docker-compose.yml`) — broker mounts
+  `./enrol-pki:/pki`, and the admin plane is **plaintext by default**
+  (`ENROL_ADMIN_INSECURE=true` in `serving/.env`). Set it `false`, then:
+  ```bash
+  cd ..                                     # serving/
+  ./enrol-broker/admin-pki.sh ./enrol-pki/broker fleet-manager
+  #   -> enrol-pki/broker/{admin-ca.crt, server.crt, server.key, clients/fleet-manager.*}
+  docker compose up -d enrol-broker         # restart so it serves mTLS (no cert error in logs)
+  mkdir -p fleet-manager/pki/broker
+  cp    enrol-pki/broker/admin-ca.crt  fleet-manager/pki/broker/
+  cp -r enrol-pki/broker/clients       fleet-manager/pki/broker/
+  ```
+- **Bench stack** (`serving/dot1x-bench/`) — broker mounts `./pki:/pki` and is
+  mTLS-by-default (cert paths hardcoded in that compose):
+  ```bash
+  ../enrol-broker/admin-pki.sh ../dot1x-bench/pki/broker fleet-manager
+  cp -r ../dot1x-bench/pki/broker ./pki/broker
+  ```
+
+Either way the manager ends up with `./pki/broker/{admin-ca.crt,
+clients/fleet-manager.{crt,key}}`. If you (re)generate a fresh admin CA, **re-copy**
+it to the manager — a stale copy won't verify. Verify the mTLS end to end:
 ```bash
-../enrol-broker/admin-pki.sh ../dot1x-bench/pki/broker fleet-manager
-cp -r ../dot1x-bench/pki/broker ./pki/broker
-# -> ./pki/broker/{admin-ca.crt, clients/fleet-manager.{crt,key}}
+curl -s --cert ./pki/broker/clients/fleet-manager.crt \
+        --key  ./pki/broker/clients/fleet-manager.key \
+        --cacert ./pki/broker/admin-ca.crt \
+        https://127.0.0.1:9443/pending ; echo    # expect [] or null (empty queue)
 ```
 
 ### 2c. Admin UI cert (Let's Encrypt DNS-01)
@@ -146,6 +174,31 @@ and the `influx-data` volume. Losing `admins.json` locks every admin out — you
 re-bootstrap with `register-admin`. Losing `enroll.key` invalidates any
 outstanding (unused) enrol links.
 
+## Troubleshooting
+
+- **`permission denied` writing the inventory on approve** — the runtime image is
+  distroless `nonroot` (uid **65532**), but a named volume is created root-owned,
+  so `/data` isn't writable. A rebuilt image pre-creates `/data` owned by nonroot
+  (a *fresh* volume inherits it), but an **already-created** volume must be fixed
+  once:
+  ```bash
+  docker volume ls | grep fleet          # find the name, e.g. fleet-manager_fleet-data
+  docker run --rm -v <vol>:/data alpine chown -R 65532:65532 /data
+  ```
+- **`authorise plane not configured (no enrol-broker wired)` in the SPA** — the
+  manager has the pillar disabled (`s.broker == nil`). Check
+  `docker compose logs fleet-manager | grep -i pillar`: a `DISABLED`/`WARNING`
+  line names the missing URL or cert file. After staging the broker PKI, the
+  container must be **recreated** (`docker compose up -d --force-recreate
+  fleet-manager`) — a plain restart won't re-enable a plane that soft-disabled at
+  boot. Also make sure only **one** manager instance is bound to `:9500`
+  (`docker ps | grep manager`); a stale parallel instance serves the old state.
+- **`TLS connect error: wrong version number` to the broker** — the broker admin
+  plane is answering in **plaintext**: it's running with `ENROL_ADMIN_INSECURE=true`
+  (the main serving-stack default). Set it `false`, ensure the broker's
+  `server.crt`/`server.key`/`admin-ca.crt` exist in its PKI mount, and restart it
+  (see step 2b).
+
 ## What is NOT yet wired (recorded intent)
 
 Per-host **boot-version assignment** (`desired.target_version`) is stored and