feat: Windows download link + self-served update feed + install tracking

Downloads/updates:
- config: DOWNLOADS_DIR (override to a mounted volume in prod).
- static.js: serves /downloads/* (installer, latest.yml, .blockmap) with range support for
  resumable + differential auto-updates; /download/windows redirects to the current .exe
  (stable link). Landing page gets a "Download the Windows desktop app" button (hidden in-app).

Install tracking (who installed the app):
- db app_installs + repos.appInstalls (upsert by install_id, fills in the user on sign-in).
- POST /api/v1/telemetry/install (records install + user once authenticated);
  GET /api/v1/admin/installs (admin: list installs with user/version/os/last-seen).
- desktop main.js: stable per-install id in userData, exposed via preload
  (bizConnectNative.installId/version/os); home.html reportInstall() posts it after login.
- e2e: +2 checks (telemetry recorded, admin sees it). 119/119. build batch20.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/db.js

@@ -321,6 +321,25 @@ CREATE TABLE IF NOT EXISTS device_tokens (
 CREATE INDEX IF NOT EXISTS idx_devtok_user ON device_tokens(user_id);
 `);
 
+// App installs (desktop/mobile clients): one row per install, associated with the user once
+// they sign in. Lets admins see who installed the app, which version, and when it was last used.
+db.exec(`
+CREATE TABLE IF NOT EXISTS app_installs (
+  id TEXT PRIMARY KEY,
+  install_id TEXT NOT NULL UNIQUE,
+  user_id TEXT,
+  user_email TEXT,
+  tenant_id TEXT,
+  platform TEXT,
+  app_version TEXT,
+  os TEXT,
+  first_seen INTEGER NOT NULL,
+  last_seen INTEGER NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_installs_tenant ON app_installs(tenant_id);
+CREATE INDEX IF NOT EXISTS idx_installs_user ON app_installs(user_id);
+`);
+
 // Favourite conversations (per user). target = 'dm:<userId>' or 'group:<groupId>'.
 db.exec(`
 CREATE TABLE IF NOT EXISTS favorites (