Aggiunge registry nodi stream e provisioning Hetzner/lab per lo scale-out.

Prepara l'architettura multi-nodo (MediaMTX+ffmpeg) con assignment URL per sessione, admin di provision/drain e provider Cloud/DNS astratti verso mltv-stream.net.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/sessions/create.rb

@@ -34,11 +34,19 @@ module Sessions
       end
 
       StreamSession.transaction do
+        session.stream_node = Streams::NodeRegistry.allocate!
         session.save!
         Scoring::Engine.ensure_score_for(session)
-        mtx = Mediamtx::Client.new
-        mtx.create_path(session)
-        log_event(session, "pairing", { created: true, platform: session.platform })
+        Mediamtx::Client.for_session(session).create_path(session)
+        log_event(
+          session,
+          "pairing",
+          {
+            created: true,
+            platform: session.platform,
+            stream_node: session.stream_node&.slug
+          }
+        )
       end
 
       if session.platform == "youtube"
@@ -46,6 +54,8 @@ module Sessions
       end
 
       session
+    rescue Streams::NodeRegistry::NoCapacityError => e
+      raise Teams::EntitlementError.new(e.message, code: "stream_capacity_exhausted")
     end
 
     private