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/admin.html

@@ -41,6 +41,39 @@
           </tbody>
         </table>
 
+        <h2 class="card-title" style="margin-top: 24px;">Email (SMTP)</h2>
+        <p class="sub">Used for "forgot password" links and battery-low
+          alerts (set per frame in its Configuration tab). Each user needs
+          an email set in their own Settings for either to reach them.</p>
+        <form method="post" action="/admin/smtp">
+          <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+          <label>SMTP server
+            <input type="text" name="smtp_host" placeholder="smtp.example.com" value="{{ smtp.smtp_host }}">
+          </label>
+          <label>Port
+            <input type="number" name="smtp_port" min="1" max="65535" value="{{ smtp.smtp_port }}">
+          </label>
+          <label>Username
+            <input type="text" name="smtp_username" autocomplete="off" value="{{ smtp.smtp_username }}">
+          </label>
+          <label>Password
+            <input type="password" name="smtp_password" autocomplete="off"
+                   placeholder="{% if smtp.smtp_password %}(unchanged -- enter a new one to replace){% else %}smtp password{% endif %}">
+          </label>
+          <label>From address
+            <input type="text" name="smtp_from_address" placeholder="[email protected]" value="{{ smtp.smtp_from_address }}">
+          </label>
+          <div class="checkbox-row">
+            <input type="checkbox" id="smtp_use_tls" name="smtp_use_tls" value="true" {% if smtp.smtp_use_tls %}checked{% endif %}>
+            <label for="smtp_use_tls">Use STARTTLS</label>
+          </div>
+          <button type="submit">Save SMTP settings</button>
+        </form>
+        <form method="post" action="/admin/smtp/test" style="margin-top: 8px;">
+          <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+          <button type="submit" class="secondary">Send test email to myself</button>
+        </form>
+
         <h2 class="card-title" style="margin-top: 24px;">Enroll a user</h2>
         <form method="post" action="/admin/users">
           <input type="hidden" name="csrf_token" value="{{ csrf_token }}">