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/README.md

@@ -0,0 +1,29 @@
+# Ping — deploy
+
+Reuses the Zitadel and SeaweedFS containers already running for Musify instead of starting new
+ones. Musify's stack must be up first.
+
+```bash
+cp deploy/.env.example deploy/.env
+# edit deploy/.env:
+#   - ZITADEL_ADMIN_PAT_DIR must point at Musify's deploy/zitadel/.output folder
+#   - S3_ACCESS_KEY / S3_SECRET_KEY must match the REAL values in Musify's own deploy/.env
+
+docker compose -f deploy/compose.yml -f deploy/compose.dev.yml up -d
+```
+
+This starts Postgres, LiveKit, and the one-shot jobs (EF Core migrations, the Zitadel
+project/apps for Ping, the `ping-storage` S3 bucket) — the usual dev loop runs the API and
+frontend from the IDE against these.
+
+To also build and run the API/frontend in Docker instead:
+
+```bash
+docker compose -f deploy/compose.yml -f deploy/compose.dev.yml --profile apps up -d --build
+```
+
+By then `zitadel-init` has already written `AUTH_CLIENT_ID` / `WEB_CLIENT_ID` into `deploy/.env`.
+
+Ports (dev): API `5211`, web `3100`, Postgres `59010`, LiveKit `7880` (signaling) / `7881` (RTC
+TCP) / `50000-50100/udp` (media). Zitadel (`8080`) and the SeaweedFS S3 API (`8333`) come from
+Musify's own stack.