Add SMTP email: password reset + per-frame battery-threshold alerts

Admin-configured SMTP (server/port/username/password/from address/
STARTTLS, a singleton server_settings row set from /admin -- not env
vars, since it's operator infrastructure a household admin sets up
once through the UI) powers two features, both requiring the relevant
user to have an email set in their own Settings:

- "Forgot password?" on /login emails a one-hour single-use reset link
  (password_reset_tokens table). The endpoint always returns the same
  generic "check your email" response regardless of whether the address
  matched an account, so it can't be used to enumerate registered users.
- A frame's Configuration tab can set a battery-alert threshold
  (Frame.battery_alert_threshold_pct, -1 = disabled); POST /frame/battery
  emails the owner the first time a report drops to or below it, then
  stays quiet for the rest of that discharge cycle (battery_alert_sent,
  reset alongside battery_history whenever the existing recharge-jump
  detection fires) -- not once per wake.

New app/mail.py wraps stdlib smtplib (no new dependency); send_email()
never raises, so a broken mail server can't 500 a battery report or a
password-reset request. Schema migration v2 adds users.email and the
two frame columns via ALTER TABLE (safe against the live, already-
populated database) plus the two new tables via the existing
create_all-based migration runner.

Verified against a real (already-migrated, real user/frame data)
database: the v1->v2 migration, admin SMTP config + test-email button,
full forgot/reset-password roundtrip (including single-use token
invalidation and the no-enumeration response), and the battery alert
firing exactly once per crossing against a hand-rolled fake SMTP
server -- all via curl end-to-end, plus the standing legacy-device
curl suite to confirm the device protocol is untouched.

server/app/static/frame_config.js

@@ -66,6 +66,26 @@ async function loadControl() {
 
 document.getElementById('take-control').addEventListener('click', takeControl);
 
+// ---- Battery alerts card ----
+
+document.getElementById('battery-alert-save').addEventListener('click', async () => {
+  const raw = document.getElementById('battery_alert_threshold_pct').value.trim();
+  const body = new URLSearchParams({
+    battery_alert_threshold_pct: raw === '' ? '-1' : raw,
+  });
+  try {
+    const resp = await fetch(`${window.FRAME_API}/config`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+      body,
+    });
+    if (!resp.ok) throw new Error(await apiError(resp));
+    showStatus(true, 'Saved.');
+  } catch (e) {
+    showStatus(false, e.message);
+  }
+});
+
 // ---- Firmware card ----
 
 document.getElementById('firmware-upload').addEventListener('click', async () => {