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

config.dev.yml

@@ -0,0 +1,7 @@
+# Development config for local Docker testing
+signing_key: "dev-signing-key-minimum-32-chars!"
+debug: true
+allow_http: true
+whitelist_hosts:
+  - localhost
+  - s3.sneak.cloud