feat(feedback): gate onboarding survey on first processing, add prompt lifecycle events (#615)

Defers the onboarding usage survey to the instance's first successful processing (the worker writes a one-time onboarding.firstProcessedAt marker and the overlay gates on it), so it reaches engaged users instead of first-landing visitors.

Replaces the two questions telemetry already answers (modality preference from tool_used, install method from instance_started) with what it can't infer: prior tool, self-host motivation, and discovery source.

Adds feedback_prompt_shown and feedback_prompt_dismissed on all five feedback surfaces (usage survey, per-job prompt, admin install card, global nav dialog, search-miss) so skip and completion rates are measurable, not just submissions. New survey strings translated into all 20 non-English locales.

tests/unit/api/settings-helpers.test.ts

@@ -5,6 +5,7 @@ import { db, schema } from "../../../apps/api/src/db/index.js";
 import {
   getSettingNumber,
   getSettingString,
+  setSettingIfAbsent,
   upsertSetting,
 } from "../../../apps/api/src/lib/settings-helpers.js";
 
@@ -117,3 +118,28 @@ describe("getSettingString", () => {
     expect(result).toBe("");
   });
 });
+
+describe("setSettingIfAbsent", () => {
+  it("writes the value when the key is absent", async () => {
+    const key = uniqueKey();
+    keysToClean.push(key);
+
+    await setSettingIfAbsent(key, "first");
+
+    const rows = await db.select().from(schema.settings).where(eq(schema.settings.key, key));
+    expect(rows).toHaveLength(1);
+    expect(rows[0].value).toBe("first");
+  });
+
+  it("keeps the first value and ignores later writes (first-write-wins)", async () => {
+    const key = uniqueKey();
+    keysToClean.push(key);
+
+    await setSettingIfAbsent(key, "first");
+    await setSettingIfAbsent(key, "second");
+
+    const rows = await db.select().from(schema.settings).where(eq(schema.settings.key, key));
+    expect(rows).toHaveLength(1);
+    expect(rows[0].value).toBe("first");
+  });
+});