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/routers/api_frames.py

@@ -78,6 +78,7 @@ def api_config_save(
     timezone: str | None = Form(None),
     firmware_update_repo_url: str | None = Form(None),
     firmware_auto_update: bool | None = Form(None),
+    battery_alert_threshold_pct: int | None = Form(None),
     frame: Frame = Depends(require_frame_control),
     db: Session = Depends(get_db),
 ):
@@ -118,6 +119,11 @@ def api_config_save(
             cfg.firmware_update_repo_url = firmware_update_repo_url.strip()
         if firmware_auto_update is not None:
             cfg.firmware_auto_update = firmware_auto_update
+        if battery_alert_threshold_pct is not None:
+            cfg.battery_alert_threshold_pct = max(-1, min(100, battery_alert_threshold_pct))
+            # A changed threshold should be able to fire again immediately,
+            # not stay suppressed by a flag set under the old value.
+            cfg.battery_alert_sent = False
         cfg.stats_config_saves += 1
     return {"status": "saved"}