modules: wire tool-runtime app credentials as own-secrets

The six modules that run a mesh-<mod> tool-runtime sidecar read an app
credential from an env var the manifest never provided, so the sidecar
crash-looped in the whole-mesh dry-run (e.g. "no Plex token — set
MESH_PLEX_TOKEN"). These are operator-set app secrets, so deliver them the
same way cloudflare-dns delivers its API token: an own-secret file mounted
read-only, with a MESH_<APP>_*_FILE env pointing at the mount, and the
runtime code preferring that file (falling back to the existing env so
nothing regresses).

- plex: own-secret token -> /run/secrets/token, MESH_PLEX_TOKEN_FILE
- bazarr: own-secret api-key -> /run/secrets/api-key, MESH_BAZARR_API_KEY_FILE
- ombi: own-secret api-key -> /run/secrets/api-key, MESH_OMBI_API_KEY_FILE
- home-assistant: own-secret token -> /run/secrets/token, MESH_HOMEASSISTANT_TOKEN_FILE
- nzbget: own-secret password -> /run/secrets/password, MESH_NZBGET_PASSWORD_FILE (URL stays plain env)
- qbittorrent: own-secret password -> /run/secrets/password, MESH_QBITTORRENT_PASSWORD_FILE (URL stays plain env)

The operator now completes each with `secret accept <node> <module> <name> --from <file>`.
tsc passes for all six.

modules/nzbget/client.ts

@@ -48,6 +48,14 @@ function meshConfig(file?: string): Record<string, string> {
   catch { return {}; }
 }
 
+/** Read a secret the mesh mounted at a file path (an own-secret delivered by `secret accept`);
+ *  absent or unreadable yields undefined so callers fall back rather than crash. */
+function readSecret(file?: string): string | undefined {
+  if (!file) return undefined;
+  try { return readFileSync(file, "utf8").trim(); }
+  catch { return undefined; }
+}
+
 export class NzbgetClient {
   readonly rpcUrl: string;
   private readonly auth: string;
@@ -66,7 +74,7 @@ export class NzbgetClient {
   static fromEnv(env: NodeJS.ProcessEnv = process.env): NzbgetClient {
     const cfg = meshConfig(env.MESH_NZBGET_CONFIG_FILE);
     const url = cfg.url ?? env.MESH_NZBGET_URL;
-    const password = cfg.password ?? env.MESH_NZBGET_PASSWORD;
+    const password = cfg.password ?? readSecret(env.MESH_NZBGET_PASSWORD_FILE) ?? env.MESH_NZBGET_PASSWORD;
     if (!url || !password) {
       throw new Error("NZBGet not configured — set MESH_NZBGET_URL and MESH_NZBGET_PASSWORD");
     }