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/templates/frame_config.html

@@ -99,6 +99,19 @@
         <button type="button" class="secondary" id="firmware-check-now">Check now</button>
         <button type="button" id="firmware-update-btn" style="display: none;">Update frame</button>
       </section>
+
+      <section class="card">
+        <h2 class="card-title">Battery alerts</h2>
+        <label>Email me when battery drops below (%)
+          <input type="number" id="battery_alert_threshold_pct" min="0" max="100"
+                 value="{% if frame.battery_alert_threshold_pct >= 0 %}{{ frame.battery_alert_threshold_pct }}{% endif %}"
+                 placeholder="disabled">
+        </label>
+        <p class="sub" style="margin-top: 8px;">Sent once per discharge cycle
+          to the frame owner's email (set in Settings) -- clear the field to
+          disable. Needs SMTP configured by an admin.</p>
+        <button type="button" class="secondary" id="battery-alert-save">Save</button>
+      </section>
     </div>
   </div>