feat: harden runtime readiness and session workflow

backend/src/runtime/readiness-manager.ts

@@ -0,0 +1,44 @@
+import type { OllamaEnsureResult, ThtRunner } from "../tht/tht-runner.js";
+
+interface ReadyEntry {
+  expiresAt: number;
+  result: OllamaEnsureResult;
+}
+
+/**
+ * Deduplicates embedding readiness checks and keeps only short-lived successes.
+ * Failures are deliberately not cached so a submit can retry after a transient outage.
+ */
+export class ReadinessManager {
+  private inFlight = new Map<string, Promise<OllamaEnsureResult>>();
+  private ready = new Map<string, ReadyEntry>();
+
+  constructor(
+    private tht: ThtRunner,
+    private timeoutSec: number,
+    private ttlMs = 60_000,
+    private now: () => number = Date.now,
+  ) {}
+
+  ensure(workspace = ""): Promise<OllamaEnsureResult> {
+    const cached = this.ready.get(workspace);
+    if (cached && cached.expiresAt > this.now()) return Promise.resolve(cached.result);
+    if (cached) this.ready.delete(workspace);
+
+    const current = this.inFlight.get(workspace);
+    if (current) return current;
+
+    const pending = this.tht.ollamaEnsure(workspace, this.timeoutSec)
+      .then((result) => {
+        if (result.ok) {
+          this.ready.set(workspace, { result, expiresAt: this.now() + this.ttlMs });
+        }
+        return result;
+      })
+      .finally(() => {
+        if (this.inFlight.get(workspace) === pending) this.inFlight.delete(workspace);
+      });
+    this.inFlight.set(workspace, pending);
+    return pending;
+  }
+}