Require signing_key at startup, add default config

- Add config validation: signing_key required, minimum 32 characters
- Server now fails to start without valid signing_key (no more runtime errors)
- Add config.example.yml with default whitelist hosts
- Copy config to /etc/pixa/config.yml in Docker image
- Update entrypoint to use --config /etc/pixa/config.yml
- Add config.dev.yml for local Docker development
- Mount dev config in make devserver

Makefile

@@ -49,7 +49,9 @@ docker-test:
 
 # Run local dev server in Docker
 devserver: docker devserver-stop
-	docker run -d --name pixad-dev -p 8080:8080 pixad:latest
+	docker run -d --name pixad-dev -p 8080:8080 \
+		-v $(CURDIR)/config.dev.yml:/etc/pixa/config.yml:ro \
+		pixad:latest
 	@echo "pixad running at http://localhost:8080"
 
 # Stop dev server