refactor!: move LLM configuration from in-app settings to CLI/env vars

- Remove `api_endpoint` from Settings model and settings UI
- Add `--llm-endpoint` / `AETHERA_LLM_ENDPOINT` and `--llm-key` /
  `AETHERA_LLM_KEY` CLI flags (endpoint is required)
- Update client constructor to accept API key parameter
- Update tests and documentation to reflect new configuration approach

BREAKING CHANGE: LLM endpoint and key must now be provided via
`AETHERA_LLM_ENDPOINT` and `AETHERA_LLM_KEY` environment variables or
CLI flags instead of the Settings page.

Dockerfile

@@ -27,4 +27,7 @@ EXPOSE 8080
 ENV AETHERA_LISTEN=0.0.0.0
 ENV AETHERA_PORT=8080
 ENV AETHERA_DATA_DIR=/app/data
+# LLM Configuration (required)
+# ENV AETHERA_LLM_ENDPOINT=https://api.example.com/v1
+# ENV AETHERA_LLM_KEY=your-api-key-here
 ENTRYPOINT ["./aethera"]