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/api.mjs

@@ -0,0 +1,35 @@
+export function apiFetch(url, options = {}) {
+  const headers = new Headers(options.headers);
+  headers.set('ngrok-skip-browser-warning', 'true');
+  return fetch(url, { ...options, headers });
+}
+
+// Read SSE over fetch so progress requests can carry the same headers as JSON.
+export async function* progressEvents(response) {
+  if (!response.ok || !response.body || !response.headers.get('content-type')?.includes('text/event-stream')) {
+    throw new Error('Could not connect to indexing progress.');
+  }
+  const reader = response.body.getReader();
+  const decoder = new TextDecoder();
+  let buffer = '', event = '', data = [];
+  try {
+    while (true) {
+      const { value, done } = await reader.read();
+      buffer += decoder.decode(value, { stream: !done });
+      let end;
+      while ((end = buffer.indexOf('\n')) >= 0) {
+        const line = buffer.slice(0, end).replace(/\r$/, '');
+        buffer = buffer.slice(end + 1);
+        if (!line) {
+          if (event === 'progress' && data.length) yield JSON.parse(data.join('\n'));
+          event = ''; data = [];
+        } else if (line.startsWith('event:')) event = line.slice(6).trim();
+        else if (line.startsWith('data:')) data.push(line.slice(5).replace(/^ /, ''));
+      }
+      if (done) break;
+    }
+  } finally {
+    await reader.cancel().catch(() => {});
+    reader.releaseLock();
+  }
+}