Server keys in etc/, bind in docker compose

- bin/gen-server-keys.sh: generate Ed25519 keypair to etc/server-service.{pub,key,env}
- main.go: read keys from file (ADMIN_PUBLIC_KEY_FILE) when env empty
- docker-compose: env_file etc/server-service.env, mount etc/
- bin/up.sh: auto-run gen-server-keys if etc/server-service.env missing
- ErrRegistrationNotConfigured for clearer 503 when keys not set
- etc/README.md, etc/.gitignore
- bin/gen-admin-key.sh for one-off key gen
- .env.example

Made-with: Cursor

docs/typescript-frontend-integration.md

@@ -68,10 +68,12 @@ Key methods:
 
 When `SERVICE_PUBLIC_KEY` (or `ADMIN_PUBLIC_KEY`) is set, users can register without an invitation:
 
-1. `GET /v1/service-key` — fetch the API public key to sign.
+1. `GET /v1/service-key` — fetch the server public key (clients use this for registration and further signed communication).
 2. Sign that key with your private key.
 3. `POST /v1/auth/register-by-signature` with `{ publicKey, signature }`.
 
+Server keys are generated with `./bin/gen-server-keys.sh` and stored in `etc/`.
+
 ## Example (TypeScript app)
 
 ```ts