feat: D3 foundation — retry schema, shared types, health client, backoff helper

- Add retry_count/next_retry_at to jobs table via guarded idempotent ALTER TABLE
  (PRAGMA table_info existence check, try/catch for safety on restart)
- Add retryCount/nextRetryAt to Job type, rowToJob(), updateJob(), resetJob()
- Add getHealth() returning structured WhisperHealth (status, gpu_name,
  vram_total_mb, model, queue_depth, model_state) distinct from
  existing checkHealth() boolean helper
- Create retryWithBackoff() pure helper in src/lib/server/retry.ts with
  exponential backoff + jitter and onAttempt callback — no persistence owned
- Tests: retry column read/write/reset, getHealth() smoke tests,
  retryWithBackoff() edge cases (success, failure, backoff timing, onAttempt)

src/tests/db.test.ts

@@ -14,6 +14,7 @@ import {
 	listJobs,
 	updateJob,
 	setJobStatus,
+	resetJob,
 	savePushSubscription,
 	getAllSubscriptions,
 	deletePushSubscription
@@ -107,6 +108,28 @@ describe('updateJob', () => {
 	it('does nothing for an unknown id', () => {
 		expect(() => updateJob({ id: 'no-such-id', title: 'Ghost' })).not.toThrow();
 	});
+
+	it('stores and reads retryCount and nextRetryAt', () => {
+		const job = createJob('src', 'retry test', 'auto');
+		// Default retry_count is 0 (DEFAULT 0 in schema)
+		expect(getJob(job.id)!.retryCount).toBe(0);
+		expect(getJob(job.id)!.nextRetryAt).toBeNull();
+
+		const nextRetry = new Date(Date.now() + 60000).toISOString();
+		updateJob({ id: job.id, retryCount: 2, nextRetryAt: nextRetry });
+		const updated = getJob(job.id)!;
+		expect(updated.retryCount).toBe(2);
+		expect(updated.nextRetryAt).toBe(nextRetry);
+	});
+
+	it('resetJob clears retry fields', () => {
+		const job = createJob('src', 'reset retry', 'auto');
+		updateJob({ id: job.id, retryCount: 3, nextRetryAt: '2026-07-08T12:00:00.000Z' });
+		resetJob(job.id);
+		const after = getJob(job.id)!;
+		expect(after.retryCount).toBe(0);
+		expect(after.nextRetryAt).toBeNull();
+	});
 });
 
 // ── setJobStatus ──────────────────────────────────────────────────────────────