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.

frontend/public/pages/settings.html

@@ -3,25 +3,6 @@
   @submit.prevent="saveSettings"
   class="p-0.5 w-full flex flex-col gap-4 pt-16 mx-auto px-4 md:px-6 max-w-6xl"
 >
-  <div>
-    <label
-      for="apiEndpoint"
-      class="block text-sm font-semibold text-primary-700"
-      >API Endpoint URL</label
-    >
-    <div class="ml-1">
-      <input
-        type="url"
-        id="apiEndpoint"
-        name="apiEndpoint"
-        x-model="settings.api_endpoint"
-        class="mt-1 p-1 block w-full rounded-md border-primary-400 shadow focus:border-secondary-500 focus:ring-secondary-500 sm:text-sm text-primary-900"
-        placeholder="https://api.example.com/v1"
-        required
-      />
-      <p class="mt-2 text-xs text-primary-500">URL of your API endpoint</p>
-    </div>
-  </div>
   <div>
     <span class="text-sm font-medium font-semibold text-primary-700"
       >Selectors</span