feat(mesh): NATS JetStream transport replacing MQTT

Replace the Mosquitto/paho mesh transport with NATS JetStream KV:
- mesh_wire.py: transport-agnostic registry (de)serialization (secret-stripped)
- nats_client.py: async CastleNATSClient — registry in the castle-registry KV
  bucket, heartbeat liveness, delete-on-stop offline, watch-driven mesh state
- rewire main/config/models/nodes; MeshStatus -> connected/nats_url
- frontend MeshStatus + MeshPanel to the new fields
- drop paho-mqtt/mqtt_client + _mqtt._tcp mDNS browse; add nats-py
- keep the secret-stripping invariant; mDNS peer discovery retained

Single-node verified; two-node parity pending the second node.

uv.lock

@@ -48,7 +48,7 @@ dependencies = [
     { name = "castle-core" },
     { name = "fastapi" },
     { name = "httpx" },
-    { name = "paho-mqtt" },
+    { name = "nats-py" },
     { name = "pydantic-settings" },
     { name = "uvicorn", extra = ["standard"] },
     { name = "zeroconf" },
@@ -67,7 +67,7 @@ requires-dist = [
     { name = "castle-core", editable = "core" },
     { name = "fastapi", specifier = ">=0.115.0" },
     { name = "httpx", specifier = ">=0.27.0" },
-    { name = "paho-mqtt", specifier = ">=2.0.0" },
+    { name = "nats-py", specifier = ">=2.9.0" },
     { name = "pydantic-settings", specifier = ">=2.0.0" },
     { name = "uvicorn", extras = ["standard"], specifier = ">=0.34.0" },
     { name = "zeroconf", specifier = ">=0.131.0" },
@@ -349,6 +349,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/70/bc/6f1c2f612465f5fa89b95bead1f44dcb607670fd42891d8fdcd5d039f4f4/markupsafe-3.0.3-cp314-cp314t-win_arm64.whl", hash = "sha256:32001d6a8fc98c8cb5c947787c5d08b0a50663d139f1305bac5885d98d9b40fa", size = 14146, upload-time = "2025-09-27T18:37:28.327Z" },
 ]
 
+[[package]]
+name = "nats-py"
+version = "2.15.0"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/02/f0/fc5e93f2b0dd14a202590ad9d30eda1955ea872039b5204357348d0f4b1e/nats_py-2.15.0.tar.gz", hash = "sha256:6622c547d9a7d2313d9c147d46c386188f4ec2c7b5c9f9a0438a4d1b55f54a93", size = 75995, upload-time = "2026-06-05T07:34:03.904Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/db/a8/b55606c7c621fb813c8ec78baf201d2c78bf6051091ec0c7ada572999e95/nats_py-2.15.0-py3-none-any.whl", hash = "sha256:9f8d36aa52a9926a88b8f1d70cf1fdce0ad387941479b500ee9ab3e51073cefd", size = 90334, upload-time = "2026-06-05T07:34:02.81Z" },
+]
+
 [[package]]
 name = "nodeenv"
 version = "1.10.0"
@@ -367,15 +376,6 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/b7/b9/c538f279a4e237a006a2c98387d081e9eb060d203d8ed34467cc0f0b9b53/packaging-26.0-py3-none-any.whl", hash = "sha256:b36f1fef9334a5588b4166f8bcd26a14e521f2b55e6b9de3aaa80d3ff7a37529", size = 74366, upload-time = "2026-01-21T20:50:37.788Z" },
 ]
 
-[[package]]
-name = "paho-mqtt"
-version = "2.1.0"
-source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/39/15/0a6214e76d4d32e7f663b109cf71fb22561c2be0f701d67f93950cd40542/paho_mqtt-2.1.0.tar.gz", hash = "sha256:12d6e7511d4137555a3f6ea167ae846af2c7357b10bc6fa4f7c3968fc1723834", size = 148848, upload-time = "2024-04-29T19:52:55.591Z" }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/c4/cb/00451c3cf31790287768bb12c6bec834f5d292eaf3022afc88e14b8afc94/paho_mqtt-2.1.0-py3-none-any.whl", hash = "sha256:6db9ba9b34ed5bc6b6e3812718c7e06e2fd7444540df2455d2c51bd58808feee", size = 67219, upload-time = "2024-04-29T19:52:48.345Z" },
-]
-
 [[package]]
 name = "pluggy"
 version = "1.6.0"