fix: back off after Reddit access denials

tests/reddit-request-coordinator.test.ts

@@ -55,4 +55,18 @@ describe('RedditRequestCoordinator', () => {
     await second;
     expect(sleeps).toEqual([16_000]);
   });
+
+  it('paces a queued request after a failed attempt without an explicit cooldown', async () => {
+    let now = 0;
+    const sleeps: number[] = [];
+    const coordinator = new RedditRequestCoordinator(6_500, {
+      now: () => now,
+      sleep: async (ms) => { sleeps.push(ms); now += ms; },
+    });
+
+    await expect(coordinator.schedule(async () => { throw new Error('403'); })).rejects.toThrow('403');
+    await coordinator.schedule(async () => undefined);
+
+    expect(sleeps).toEqual([6_500]);
+  });
 });