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/internal/credentials.py

@@ -1,30 +1,73 @@
 """
-Manages the persisted node API key.
+Manages the persisted node API key, plus the local-auth signing material used
+by app/internal/auth.py.
 
-The key is provisioned by the C2 server after an admin approves the node.
+The API key is provisioned by the C2 server after an admin approves the node.
 It arrives via MQTT and is saved to /configs/credentials.json so it survives
 container restarts.
+
+The scrypt salt and session-signing secret are generated locally on first boot
+(never provisioned externally) and persisted the same way, so dashboard
+sessions survive a container restart instead of forcing every operator to
+re-login whenever the node restarts.
 """
 import json
+import secrets
 from pathlib import Path
 from app.config import settings
 from app.internal.logger import logger
 
 _CREDS_FILE = Path(settings.config_path) / "credentials.json"
 _api_key: str | None = None
+_auth_salt: bytes | None = None
+_session_secret: bytes | None = None
 
 
 def load() -> None:
     """Load persisted credentials from disk on startup."""
-    global _api_key
+    global _api_key, _auth_salt, _session_secret
     if _CREDS_FILE.exists():
         try:
             data = json.loads(_CREDS_FILE.read_text())
             _api_key = data.get("api_key")
+            if data.get("auth_salt"):
+                _auth_salt = bytes.fromhex(data["auth_salt"])
+            if data.get("session_secret"):
+                _session_secret = bytes.fromhex(data["session_secret"])
             if _api_key:
                 logger.info("Node credentials loaded from disk.")
         except Exception as e:
             logger.warning(f"Could not read credentials file: {e}")
+    _ensure_auth_material()
+
+
+def _ensure_auth_material() -> None:
+    """Generate (once) and persist the local-auth salt + session secret."""
+    global _auth_salt, _session_secret
+    changed = False
+    if _auth_salt is None:
+        _auth_salt = secrets.token_bytes(16)
+        changed = True
+    if _session_secret is None:
+        _session_secret = secrets.token_bytes(32)
+        changed = True
+    if changed:
+        _write()
+        logger.info("Generated local-auth signing material (first boot).")
+
+
+def get_auth_salt() -> bytes:
+    """Scrypt salt for dashboard password hashing — generated once, persisted."""
+    if _auth_salt is None:
+        _ensure_auth_material()
+    return _auth_salt  # type: ignore[return-value]
+
+
+def get_session_secret() -> bytes:
+    """HMAC key used to sign dashboard session cookies — generated once, persisted."""
+    if _session_secret is None:
+        _ensure_auth_material()
+    return _session_secret  # type: ignore[return-value]
 
 
 def get_api_key() -> str | None:
@@ -34,6 +77,15 @@ def get_api_key() -> str | None:
 def save_api_key(key: str) -> None:
     global _api_key
     _api_key = key
-    _CREDS_FILE.parent.mkdir(parents=True, exist_ok=True)
-    _CREDS_FILE.write_text(json.dumps({"api_key": key}))
+    _write()
     logger.info("Node API key saved to disk.")
+
+
+def _write() -> None:
+    _CREDS_FILE.parent.mkdir(parents=True, exist_ok=True)
+    data: dict = {"api_key": _api_key}
+    if _auth_salt is not None:
+        data["auth_salt"] = _auth_salt.hex()
+    if _session_secret is not None:
+        data["session_secret"] = _session_secret.hex()
+    _CREDS_FILE.write_text(json.dumps(data))