feat: wire chain clients into AppState (feature-gated)

Add ChainClients (infra/chains.rs): per-chain data-source handles built once at
startup from config and shared via AppState. Each client is present only when
its chain feature flag is on, and a malformed chain endpoint now fails startup
(fail-closed) rather than the first request. Cloneable (grin's session state is
shared via Arc) so it lives in the cloneable AppState.

Foundation for the wallet-handlers phase — handlers will read these clients
from State. No endpoints yet (openapi unchanged).

src/main.rs

@@ -9,7 +9,8 @@ use std::sync::Arc;
 use sqlx::postgres::PgPoolOptions;
 
 use smirk_backend_core::{
-    build_router, config::Config, core::session::SessionManager, infra::db::Database, AppState,
+    build_router, config::Config, core::session::SessionManager, infra::chains::ChainClients,
+    infra::db::Database, AppState,
 };
 
 #[tokio::main]
@@ -33,12 +34,14 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         config.secrets.ip_salt.clone(),
     );
     let sessions = SessionManager::new(&config.auth.jwt_secret, config.auth.jwt_expiry_hours);
+    let chains = ChainClients::from_config(&config)?;
 
     let addr = format!("{}:{}", config.server_host, config.server_port);
     let state = Arc::new(AppState {
         config,
         db,
         sessions,
+        chains,
         web_challenges: Arc::default(),
     });