feat(ai): curated OpenRouter model picker for hybrid backend

Add OPENROUTER_ALLOWED_MODELS env var and GET /insights/openrouter/models
endpoint returning the curated list verbatim. Drop the live capability
precheck in hybrid mode — trust the operator's allowlist; bad ids surface
as a chat-call error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -1355,6 +1355,7 @@ fn main() -> std::io::Result<()> {
                 .service(ai::delete_insight_handler)
                 .service(ai::get_all_insights_handler)
                 .service(ai::get_available_models_handler)
+                .service(ai::get_openrouter_models_handler)
                 .service(ai::rate_insight_handler)
                 .service(ai::export_training_data_handler)
                 .service(libraries::list_libraries)