feat: Tailscale sync jobs

Adds a background job system that polls the Tailscale API on a configurable
interval and syncs tailscale status and IPs to instances by hostname match.

- New config table (key/value) in SQLite for persistent server-side settings
- New server/jobs.js: runTailscaleSync + restartJobs scheduler
- GET/PUT /api/config — read and write Tailscale settings; API key masked as **REDACTED** on GET
- POST /api/jobs/tailscale/run — immediate manual sync
- Settings modal: new Tailscale Sync section with enable toggle, tailnet, API key, poll interval, Save + Run Now buttons, last-run status

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/db.test.js

@@ -3,6 +3,7 @@ import {
   _resetForTest,
   getInstances, getInstance, getDistinctStacks,
   createInstance, updateInstance, deleteInstance, importInstances, getInstanceHistory,
+  getConfig, setConfig,
 } from '../server/db.js'
 
 beforeEach(() => _resetForTest());
@@ -269,3 +270,32 @@ describe('test environment boot isolation', () => {
     expect(getInstances()).toEqual([]);
   });
 });
+
+// ── getConfig / setConfig ─────────────────────────────────────────────────────
+
+describe('getConfig / setConfig', () => {
+  it('returns defaultVal when key does not exist', () => {
+    expect(getConfig('missing', 'fallback')).toBe('fallback');
+  });
+
+  it('returns empty string by default', () => {
+    expect(getConfig('missing')).toBe('');
+  });
+
+  it('stores and retrieves a value', () => {
+    setConfig('tailscale_api_key', 'tskey-test');
+    expect(getConfig('tailscale_api_key')).toBe('tskey-test');
+  });
+
+  it('overwrites an existing key', () => {
+    setConfig('tailscale_enabled', '0');
+    setConfig('tailscale_enabled', '1');
+    expect(getConfig('tailscale_enabled')).toBe('1');
+  });
+
+  it('config is cleared by _resetForTest', () => {
+    setConfig('tailscale_api_key', 'tskey-test');
+    _resetForTest();
+    expect(getConfig('tailscale_api_key')).toBe('');
+  });
+});