Authenticate the node dashboard, and the broker connection per node

Two unauthenticated surfaces closed on the edge node.

Dashboard and API: the local dashboard and every /api/* route were open to
anything on the node's LAN. Adds a login page plus session-cookie auth for
the browser, and cookie-or-Basic for the API so scripted callers stay
possible. Passwords are hashed with stdlib scrypt (no new dependency, this
runs on a Pi) and compared in constant time; the salt and session-signing
secret persist in credentials.json. Startup warns while the default password
is still in place. No non-browser callers of the node API exist today
(C2 talks to nodes over MQTT and nodes call C2 outbound), so nothing breaks.

Adds python-multipart, which FastAPI's Form() needs for the login POST and
which was missing from requirements entirely.

MQTT: nodes authenticated with a shared drb-node password, and the broker
ACL keyed off %c — the client-supplied client id — so any holder of that one
password could claim another node's topic namespace. Nodes now connect as
username=<node_id>, password=<their C2-issued api_key>, which mosquitto's
dynamic-security plugin checks, with the ACL keyed off the authenticated %u.
TLS is gated on MQTT_TLS and uses default CA verification.

The old key_request MQTT path stays in place behind TODO(mqtt-cutover)
markers as the fallback until the cutover is proven; a node with no api_key
on disk logs a clear repeated refusal rather than spinning.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

drb-edge-node/app/routers/api.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter, HTTPException, Body
+from fastapi import APIRouter, Depends, HTTPException, Body
 from typing import Optional
 import asyncio
 import httpx
@@ -11,8 +11,14 @@ from app.internal.discord_radio import radio_bot
 from app.internal.metadata_watcher import metadata_watcher
 from app.internal import credentials
 from app.internal.mqtt_manager import mqtt_manager
+from app.internal import auth
 
-router = APIRouter(prefix="/api", tags=["api"])
+# Every route in this router requires auth — a valid dashboard session cookie
+# or HTTP Basic (see app/internal/auth.py). No exemption exists for any route
+# here: there is no health/liveness endpoint in this file or anywhere else in
+# the edge node (confirmed against source — no docker healthcheck references
+# one either), so nothing needs to stay open for a container healthcheck.
+router = APIRouter(prefix="/api", tags=["api"], dependencies=[Depends(auth.require_auth)])
 
 
 @router.get("/status")