feat(db): migrate from lowdb to SQLite with repos pattern

- Add modular DB layer (adapters, migrations, repos, helpers)
- Replace localDb/usageDb/requestDetailsDb monoliths with repos
- Add Tailscale tunnel integration & status check API
- Add /api/cli-tools/all-statuses aggregated endpoint
- Add settingsStore (Zustand) and mitm/dbReader
- Add DB unit tests (benchmark, concurrent, migration, vs-lowdb)

src/lib/db/helpers/jsonCol.js

@@ -0,0 +1,9 @@
+export function parseJson(str, fallback = null) {
+  if (str == null) return fallback;
+  if (typeof str !== "string") return str;
+  try { return JSON.parse(str); } catch { return fallback; }
+}
+
+export function stringifyJson(value) {
+  return JSON.stringify(value ?? null);
+}