feat: GET /api/v1/capabilities (client feature discovery)

Public, unauthenticated description of what this deployment offers, so the
wallet can adapt to the instance it's pointed at — grey out disabled chains and
features, and pick the right network. This is the client-facing half of the
open-core/feature-flag design: every capability is a config switch, surfaced
here. No secrets — only on/off flags and public network names.

Response: { version, chains: { btc/ltc/xmr/wow/grin: { enabled, network } },
features: { grin_relay, prices, nostr_identity, tips } }. UTXO chains carry
their configured network; CryptoNote/Grin report null. On the normal rate-limit
tier. Registered in openapi (system tag); 1 L1 test (public, well-shaped).

tests/capabilities.rs

@@ -0,0 +1,32 @@
+//! L1 integration: the public capabilities endpoint.
+
+mod common;
+
+use axum::http::StatusCode;
+
+#[tokio::test]
+async fn capabilities_is_public_and_well_shaped() {
+    let app = require_app!();
+    // No token required — the wallet reads this before/without login.
+    let (status, body) = app.request("GET", "/api/v1/capabilities", None, None).await;
+    assert_eq!(status, StatusCode::OK);
+
+    assert!(body["version"].as_str().is_some(), "version present");
+    // Every chain reports an `enabled` boolean.
+    for chain in ["btc", "ltc", "xmr", "wow", "grin"] {
+        assert!(
+            body["chains"][chain]["enabled"].is_boolean(),
+            "chain {chain} reports enabled"
+        );
+    }
+    // UTXO chains carry a network; CryptoNote/Grin don't.
+    assert!(body["chains"]["btc"]["network"].as_str().is_some());
+    assert!(body["chains"]["xmr"]["network"].is_null());
+    // Feature flags are present.
+    for feat in ["grin_relay", "prices", "nostr_identity", "tips"] {
+        assert!(
+            body["features"][feat].is_boolean(),
+            "feature {feat} reports a boolean"
+        );
+    }
+}