radarr: reach nzbget, qbittorrent and jackett through the mesh

radarr reached its download clients as nzbget:6789 and qbittorrent:8112 and its indexers
through jackett:9117 or indexers.zurag.be - HAL container names and a public route,
typed into its database by hand. Nothing on the mesh answers those names.

It now requires nzbget-api, qbittorrent-api and jackett-api. A run-once step
(downloads/, declared last, restart-on its bindings, credentials and settings) writes
host, port, TLS, base path, user and credential into radarr through radarr's own API:

- A download client is the mesh's when its name is downloads.<provision>.name and its
  kind the provider's; it is registered when missing. A jackett feed is the mesh's when
  its host is the bound one, one the step bound before, or one in
  downloads.jackett-api.adopt-hosts; the jackett indexers in
  downloads.jackett-api.indexers are registered when missing. Every other entry is left
  alone, and nothing is ever deleted.
- Only connection fields, only when they differ. The stored password is masked, so the
  app tests the entry with the credential it holds; only if that fails is the delivered
  one written. Categories, priorities and "enabled" are never touched.
- Each credential is tried against its provider first. A minted value (nothing accepted
  yet) is never written; the step exits 1 naming the exact secret accept.

The step is byte-identical in sonarr, radarr, lidarr and bookshelf (the same Servarr
API, v3 or v1): each module builds from its own directory, so each carries a copy, and
test/downloads.test.ts fails if a sibling's copy differs.

Verified: strict typecheck, the Dockerfile build, 14 unit tests; and the compiled step
against fresh pinned sonarr/radarr/lidarr/bookshelf with throwaway nzbget, qBittorrent
and jackett - minted credentials refused with nothing written, accepted ones registered
and tested by the app, a migration-shaped radarr repointed, reruns unchanged.

modules/radarr/module.json

@@ -97,9 +97,66 @@
         "MESH_RADARR_CONFIG_DIR": "/var/lib/radarr/config"
       },
       "artifact": "runtime"
+    },
+    {
+      "id": "downloads-config",
+      "type": "file",
+      "path": "${dir:state}/downloads.json",
+      "mode": "0600",
+      "content": "{\n  \"node\": \"${machine:name}\",\n  \"downloads\": {\n    \"nzbget-api\": {\n      \"name\": \"nzbget\"\n    },\n    \"qbittorrent-api\": {\n      \"name\": \"qbittorrent\"\n    },\n    \"jackett-api\": {\n      \"adopt-hosts\": [],\n      \"indexers\": []\n    }\n  }\n}\n",
+      "merge": "json"
+    },
+    {
+      "id": "downloads-memory",
+      "type": "directory",
+      "mode": "0700"
+    },
+    {
+      "id": "downloads",
+      "type": "container",
+      "name": "mesh-radarr-downloads",
+      "network": "host",
+      "run-once": true,
+      "volumes": [
+        "${dir:state}/downloads.json:/run/downloads/downloads.json:ro",
+        "${dir:state}/nzbget-api.json:/run/downloads/nzbget-api.json:ro",
+        "${dir:state}/nzbget-api.secret:/run/downloads/nzbget-api.secret:ro",
+        "${dir:state}/qbittorrent-api.json:/run/downloads/qbittorrent-api.json:ro",
+        "${dir:state}/qbittorrent-api.secret:/run/downloads/qbittorrent-api.secret:ro",
+        "${dir:state}/jackett-api.json:/run/downloads/jackett-api.json:ro",
+        "${dir:state}/jackett-api.secret:/run/downloads/jackett-api.secret:ro",
+        "${dir:config}:/var/lib/radarr/config:ro",
+        "${dir:downloads-memory}:/var/lib/downloads"
+      ],
+      "env": {
+        "MESH_DOWNLOADS_APP": "radarr",
+        "MESH_DOWNLOADS_API": "v3",
+        "MESH_DOWNLOADS_APP_URL": "http://127.0.0.1:${port:7878}",
+        "MESH_DOWNLOADS_APP_CONFIG": "/var/lib/radarr/config/config.xml",
+        "MESH_DOWNLOADS_DIR": "/run/downloads",
+        "MESH_DOWNLOADS_SETTINGS": "/run/downloads/downloads.json",
+        "MESH_DOWNLOADS_MEMORY": "/var/lib/downloads/memory.json"
+      },
+      "args": [
+        "run",
+        "/app/modules/radarr/dist/downloads/index.js"
+      ],
+      "restart-on": [
+        "downloads-config",
+        "bound-nzbget-api",
+        "secret-nzbget-api",
+        "bound-qbittorrent-api",
+        "secret-qbittorrent-api",
+        "bound-jackett-api",
+        "secret-jackett-api"
+      ],
+      "artifact": "runtime"
     }
   ],
   "requires": [
+    "jackett-api",
+    "nzbget-api",
+    "qbittorrent-api",
     "route"
   ],
   "contributes": {
@@ -109,7 +166,15 @@
     }
   },
   "binds": {
-    "route": "${dir:state}/route.json"
+    "route": "${dir:state}/route.json",
+    "nzbget-api": "${dir:state}/nzbget-api.json",
+    "qbittorrent-api": "${dir:state}/qbittorrent-api.json",
+    "jackett-api": "${dir:state}/jackett-api.json"
+  },
+  "secrets": {
+    "nzbget-api": "${dir:state}/nzbget-api.secret",
+    "qbittorrent-api": "${dir:state}/qbittorrent-api.secret",
+    "jackett-api": "${dir:state}/jackett-api.secret"
   },
   "build": {
     "on": [