chore: initial foundation — server skeleton, autodocs, CI

- minimal Axum server exposing GET /health
- OpenAPI contract generated from handlers (gen_openapi -> openapi.json),
  drift-gated in CI so the spec can never silently diverge from the code
- CI: secret-scan + gitleaks, openapi drift gate, fmt/clippy/build/test/doc-test, cargo-deny
- Cargo.lock tracked for reproducible builds; publish=false; pinned toolchain
- README, SECURITY.md, deny.toml, .env.example

src/api/health.rs

@@ -0,0 +1,27 @@
+//! Health / liveness endpoint.
+
+use axum::Json;
+use serde::Serialize;
+
+/// Liveness response.
+#[derive(Serialize, utoipa::ToSchema)]
+pub struct HealthResponse {
+    /// Always `"ok"` when the service is reachable.
+    pub status: String,
+}
+
+/// Liveness probe.
+///
+/// Returns `200` with `{ "status": "ok" }` whenever the server is up. Used by
+/// load balancers and uptime checks; requires no authentication.
+#[utoipa::path(
+    get,
+    path = "/health",
+    responses((status = 200, description = "Service is up", body = HealthResponse)),
+    tag = "system"
+)]
+pub async fn health() -> Json<HealthResponse> {
+    Json(HealthResponse {
+        status: "ok".to_string(),
+    })
+}

src/api/mod.rs

@@ -0,0 +1,4 @@
+//! HTTP API layer: handlers and the OpenAPI assembly.
+
+pub mod health;
+pub mod openapi;

src/api/openapi.rs

@@ -0,0 +1,21 @@
+//! OpenAPI specification assembly.
+//!
+//! `ApiDoc` aggregates the annotated handlers into the spec that is the single
+//! source of truth for the API contract and the wallet's generated TypeScript
+//! client. Add a path here once its handler carries `#[utoipa::path(...)]` and
+//! its request/response types derive `utoipa::ToSchema`.
+
+use utoipa::OpenApi;
+
+#[derive(OpenApi)]
+#[openapi(
+    info(
+        title = "smirk-backend-core",
+        version = "0.3.0",
+        description = "Open, self-hostable backend for the Smirk non-custodial multi-chain wallet."
+    ),
+    paths(crate::api::health::health),
+    components(schemas(crate::api::health::HealthResponse)),
+    tags((name = "system", description = "Service health and metadata."))
+)]
+pub struct ApiDoc;