jackett: provide its Torznab API as jackett-api

sonarr, radarr, lidarr and bookshelf reached jackett as http://jackett:9117 (a HAL
container name) or https://indexers.zurag.be (its public route), typed into each app by
hand. The mesh has neither: an app now requires jackett-api and its downloads step writes
the bound address into the app.

Serves scheme, port and url-base; `at` and the machine port come from the binding. Mesh
scope, like sonarr-api: an indexer proxy shares no files with its consumers.

The pair credential is jackett's one API key. The mesh cannot mint it, so the operator
accepts it per consumer pair (ADR 0092), as #156 does for sonarr-api; a consumer's step
refuses a minted value and names the accept.

modules/jackett/module.json

@@ -1,6 +1,19 @@
 {
   "module": "jackett",
   "version": "1",
+  "provides": [
+    {
+      "name": "jackett-api",
+      "scope": "mesh"
+    }
+  ],
+  "serves": {
+    "jackett-api": {
+      "scheme": "http",
+      "port": 9117,
+      "url-base": ""
+    }
+  },
   "capabilities": [
     "container-runtime"
   ],
@@ -10,7 +23,7 @@
       "port": 9117,
       "protocol": "tcp",
       "from": "mesh",
-      "why": "the indexer proxy: its web UI, and the Torznab feeds the *arr apps search through"
+      "why": "the indexer proxy: its web UI, and the Torznab feeds the *arr apps search through, which other modules reach as jackett-api"
     }
   ],
   "resources": [