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>

server/db.js

@@ -53,6 +53,11 @@ function createSchema() {
       changed_at TEXT    NOT NULL DEFAULT (datetime('now'))
     );
     CREATE INDEX IF NOT EXISTS idx_history_vmid ON instance_history(vmid);
+
+    CREATE TABLE IF NOT EXISTS config (
+      key   TEXT PRIMARY KEY,
+      value TEXT NOT NULL DEFAULT ''
+    );
   `);
 }
 
@@ -187,6 +192,18 @@ export function getAllHistory() {
   return db.prepare('SELECT * FROM instance_history ORDER BY vmid, changed_at').all();
 }
 
+export function getConfig(key, defaultVal = '') {
+  const row = db.prepare('SELECT value FROM config WHERE key = ?').get(key);
+  return row ? row.value : defaultVal;
+}
+
+export function setConfig(key, value) {
+  db.prepare(
+    `INSERT INTO config (key, value) VALUES (?, ?)
+     ON CONFLICT(key) DO UPDATE SET value = excluded.value`
+  ).run(key, String(value));
+}
+
 // ── Test helpers ──────────────────────────────────────────────────────────────
 
 export function _resetForTest() {