Add auto-interconnect service for *arr stack

Systemd oneshot that runs after all services start and configures:
- Prowlarr → Sonarr (TV indexers, full sync)
- Prowlarr → Radarr (movie indexers, full sync)
- Sonarr → qBittorrent (download client, category: tv-sonarr)
- Radarr → qBittorrent (download client, category: radarr)
- Bazarr → Sonarr (subtitle management for TV)
- Bazarr → Radarr (subtitle management for movies)

Fully idempotent — checks for existing connections before creating.
API keys extracted from each app's config files at runtime.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

common.nix

@@ -31,6 +31,7 @@
     ./services/cloudflare-ddns.nix
     ./services/fail2ban.nix
     ./services/homepage.nix
+    ./services/arr-interconnect.nix
   ];
 
   ### Make build time quicker