Raise alerts: failed or paused prints, and AI detections

Nothing subscribed to channel:print_alerts and nothing watched a printer
fail, so an operator away from the screen found out when they came back to
a ruined plate. Alerts are now rows in printer_alerts, listed at
GET /alerts (open=true for what still needs a person) and cleared with
POST /alerts/:id/acknowledge, which is operator-only and keeps the first
acknowledger on the record.

Two sources. Telemetry: a change into error is a failed print if the printer
was printing or paused, otherwise a printer error; a print the printer
paused itself (how an AMS fault looks) is announced too. Transitions, not
levels - a printer sitting in error is one alert, not one per frame - and a
connection's first sight of a printer raises nothing, so a gateway that
reconnects does not re-announce known faults. AI: the worker's Valkey
publications are stored when confidence is at least ALERT_MIN_CONFIDENCE
(default 0.6); the worker deliberately does not threshold, so the policy
lives here. A bad message costs one message, not the listener.

e2e-authz.sh gains the scoping/ack cases; new e2e-alerts.sh drives a stand-in
gateway over the real /ws/edge/v1 and the real Valkey channel (checked to
fail when the derivation is broken).

drizzle/meta/_journal.json

@@ -29,6 +29,13 @@
       "when": 1792399397819,
       "tag": "0003_colorful_electro",
       "breakpoints": true
+    },
+    {
+      "idx": 4,
+      "version": "7",
+      "when": 1793285676878,
+      "tag": "0004_printer_alerts",
+      "breakpoints": true
     }
   ]
 }