Phase 3: MQTT transport — retained config, status/telemetry, LWT presence, TLS/ACL design

- internal/mqtt: publish each device's desired state retained on
  restrict/<device>/config; subscribe to restrict/+/status and
  restrict/+/telemetry and write reports to SQLite; publish on every state
  change, republish on connect and every 15 minutes from SQLite, so the broker
  is repaired from the source of truth.
- store: presence (devices.online and last_seen, audited only on transitions),
  telemetry upsert per (device, day, app, domain) via migration 002, allow and
  block lists read into the config payload, device lookup by topic.
- config: MQTT settings. TLS plus per-device credentials are required; the
  pre-TLS shared broker needs an explicit CONTROL_MQTT_INSECURE escape hatch.
- web: republish config after lock, unlock, expiry, create and edit; broker card
  on the status partial.
- deploy/mosquitto: TLS listener, topic ACL, credential provisioning and a
  verifier that proves the policy on a throwaway broker. Not applied to the
  shared Mosquitto on .13 — that change needs Sam's approval.
- cmd/control: mqtt-clients prints the MQTT identity of every device.
- paho.mqtt.golang v1.5.1, which pulls golang.org/x/net and with it the
  golang.org/x/crypto bump to v0.42.0.

Verified on .13: retained config delivered to a fresh subscriber; a client
killed without DISCONNECT drove the broker's retained will and the service
flipped the device offline; telemetry landed in SQLite; make mqtt-verify passed
13/13; the service connected over TLS with credentials and a CA.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/store/migrations/002_telemetry_unique.sql

@@ -0,0 +1,13 @@
+-- Migration 002 — Phase 3. Telemetry reports arrive over MQTT and repeat.
+--
+-- A device reports a rolling daily total ("Spotify: 600 s today"), not a delta,
+-- so one row per (device, day, app, domain) is the correct shape: a repeat
+-- report replaces the previous total instead of adding to it. Devices may omit
+-- the app or the domain, so both default to '' — SQLite treats NULLs in a
+-- UNIQUE index as distinct, which would defeat the upsert.
+
+UPDATE telemetry SET app = '' WHERE app IS NULL;
+UPDATE telemetry SET domain = '' WHERE domain IS NULL;
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_telemetry_daily
+    ON telemetry(device_id, day, app, domain);