Add Go HTTP/SSE API server with embedded frontend

Implements the API routes (jobs, progress events, snapshots, search,
entities, source delivery with release) to net/http and serves the
static frontend from the same binary on the same origin.

web/config.js

@@ -0,0 +1,4 @@
+window.CODENAVIGATOR_CONFIG = {
+  // Empty means same origin: the Go server serves both the frontend and the API.
+  backendUrl: '',
+};