Add Ping backend

.NET 10 solution for Ping, a Discord style chat app: Domain, Application
and Infrastructure projects plus the Api host. SignalR for chat and
presence, LiveKit for call tokens, EF Core on Postgres, S3 compatible
storage for attachments, and OIDC auth shared with the Musify Zitadel
instance. Includes the Compose stack for local development.

backend/deploy/compose.dev.yml

@@ -0,0 +1,39 @@
+name: ping-dev
+
+# Development overlay: publishes the infrastructure ports to the host so the .NET apps and
+# the web frontend can run from the IDE.
+#
+#   docker compose -f deploy/compose.yml -f deploy/compose.dev.yml up -d
+#   docker compose -f deploy/compose.yml -f deploy/compose.dev.yml --profile apps up -d --build
+
+x-host-alias: &host-alias
+  - "host.docker.internal:host-gateway"
+
+services:
+  postgres:
+    ports:
+      - "${POSTGRES_HOST_PORT}:5432"
+
+  livekit:
+    ports:
+      - "7880:7880" # HTTP / WebSocket signaling
+      - "7881:7881" # RTC TCP fallback
+      - "50000-50100:50000-50100/udp" # WebRTC media
+
+  zitadel-init:
+    extra_hosts: *host-alias
+    volumes:
+      - ./.env:/workspace/.env
+
+  seaweedfs-init:
+    extra_hosts: *host-alias
+
+  api:
+    ports:
+      - "5211:5211"
+    extra_hosts: *host-alias
+
+  web:
+    ports:
+      - "3100:3100"
+    extra_hosts: *host-alias