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/ui.py

@@ -1,18 +1,64 @@
 from pathlib import Path
-from fastapi import APIRouter
-from fastapi.responses import HTMLResponse
+from typing import Optional
+
+from fastapi import APIRouter, Depends, Form
+from fastapi.responses import HTMLResponse, RedirectResponse
+
+from app.internal import auth
 
 router = APIRouter(tags=["ui"])
 
 _TEMPLATE = Path(__file__).parent.parent / "templates" / "index.html"
 _SCANNER_TEMPLATE = Path(__file__).parent.parent / "templates" / "scanner.html"
+_LOGIN_TEMPLATE = Path(__file__).parent.parent / "templates" / "login.html"
+
+
+@router.get("/login", response_class=HTMLResponse)
+async def login_page(error: Optional[str] = None):
+    html = _LOGIN_TEMPLATE.read_text()
+    banner = (
+        '<div class="error">Invalid username or password.</div>' if error else ""
+    )
+    return html.replace("<!--ERROR_BANNER-->", banner)
+
+
+@router.post("/login")
+async def login_submit(username: str = Form(...), password: str = Form(...)):
+    if not auth.verify_credentials(username, password):
+        return RedirectResponse("/login?error=1", status_code=303)
+
+    token = auth.create_session_token(username)
+    resp = RedirectResponse("/", status_code=303)
+    resp.set_cookie(
+        auth.SESSION_COOKIE_NAME,
+        token,
+        max_age=auth.SESSION_TTL_SECONDS,
+        httponly=True,
+        samesite="lax",
+        # No TLS termination on this port (LAN dashboard on :80) — `secure`
+        # would make the cookie never get sent at all.
+        secure=False,
+    )
+    return resp
+
+
+@router.post("/logout")
+@router.get("/logout")
+async def logout():
+    resp = RedirectResponse("/login", status_code=303)
+    resp.delete_cookie(auth.SESSION_COOKIE_NAME)
+    return resp
 
 
 @router.get("/", response_class=HTMLResponse)
-async def index():
+async def index(authed: bool = Depends(auth.require_session)):
+    if not authed:
+        return RedirectResponse("/login")
     return _TEMPLATE.read_text()
 
 
 @router.get("/scanner", response_class=HTMLResponse)
-async def scanner():
+async def scanner(authed: bool = Depends(auth.require_session)):
+    if not authed:
+        return RedirectResponse("/login")
     return _SCANNER_TEMPLATE.read_text()