Initial scaffold: Elsa 3.5 custom API server with sample Shout activity

- src/Elsa.CustomServer: minimal ASP.NET Core 8 host referencing Elsa 3.5 NuGets
- Activities auto-registered via AddActivitiesFrom<Program>() — drop new files under Activities/
- Sqlite persistence for workflow definitions + runtime
- Multi-stage Dockerfile targeting linux/amd64, ASP.NET 8 runtime, port 8080
- Forwarded-headers configured so URLs come out https:// behind Traefik
- CORS wide-open so the standalone Studio (different origin) can talk to us

README.md

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+# Elsa Custom Server
+
+Custom Elsa Workflows API server. Extends stock Elsa 3.x with our own activities.
+
+## Where activities live
+
+`src/Elsa.CustomServer/Activities/*.cs` — anything marked `[Activity(...)]` under this
+folder is auto-registered via `AddActivitiesFrom<Program>()` in `Program.cs`.
+
+To add a new node type in the Studio designer:
+
+1. Drop a new file under `Activities/`.
+2. Inherit from `CodeActivity`, `CodeActivity<T>`, or `Activity` for triggerable ones.
+3. Rebuild and redeploy — Studio picks it up automatically the next time it fetches
+   the activity descriptors.
+
+Example — see `Activities/Shout.cs`.
+
+## Local run
+
+```sh
+dotnet run --project src/Elsa.CustomServer
+```
+
+Listens on http://localhost:8080; API root is `/elsa/api`.
+
+## Docker
+
+```sh
+docker build -t elsa-custom-server .
+docker run --rm -p 8080:8080 \
+  -e Identity__Tokens__SigningKey="$(openssl rand -base64 64)" \
+  elsa-custom-server
+```
+
+## Deployment
+
+Coolify builds this image from the Dockerfile at repo root when the app is created
+with `build_pack: "dockerfile"`. The Studio (separate container running the stock
+`elsaworkflows/elsa-studio-v3` image) needs its `BACKEND__URL` env var pointed at
+this server's `/elsa/api`.