docs(admin): say which providers the server-key base URL accepts

The operator card offered a free-text base URL with an OpenAI placeholder
and no word about what else it takes, so an Anthropic endpoint looked
unsupported even though the field accepts that host. One line beside the
existing hints, in all seven locales, plus the changelog entry.

src/components/admin/ai-server-key-section.tsx

@@ -34,6 +34,10 @@ import { PasswordInput } from "./_shared";
  *     key here never silently forwards anyone's health data.
  *   - The base URL is locked server-side to HTTPS + a hostname allowlist
  *     (`ADMIN_AI_BASE_URL_ALLOWLIST` extends it).
+ *   - The base URL also picks the wire: an `anthropic.com` host is called on
+ *     Anthropic's Messages API, every other host on the OpenAI-compatible one
+ *     (`resolveAdminProvider` in `src/lib/ai/provider.ts`). `baseUrlHint` is
+ *     that sentence for the operator.
  */
 
 interface AiServerKeyResponse {
@@ -113,6 +117,7 @@ export function AiServerKeySection() {
         <div className="space-y-2 text-sm">
           <p>{t("admin.aiServerKey.byokHint")}</p>
           <p>{t("admin.aiServerKey.consentHint")}</p>
+          <p>{t("admin.aiServerKey.baseUrlHint")}</p>
         </div>
 
         <div className="space-y-1.5">