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/migration.py

@@ -19,7 +19,7 @@ from sqlalchemy import select, text
 
 from . import config
 from .db import SessionLocal, engine
-from .models import Base, BatteryLog, Frame
+from .models import Base, BatteryLog, Frame, ServerSettings
 
 logger = logging.getLogger(__name__)
 
@@ -28,8 +28,20 @@ def _migration_1(conn) -> None:
     Base.metadata.create_all(bind=conn)
 
 
+def _migration_2(conn) -> None:
+    """Adds email (users) and battery-alert threshold (frames) columns,
+    plus the new server_settings/password_reset_tokens tables. ALTER
+    TABLE ADD COLUMN with a default is safe on SQLite against a live,
+    already-populated database -- existing rows just get the default."""
+    conn.execute(text("ALTER TABLE users ADD COLUMN email TEXT NOT NULL DEFAULT ''"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN battery_alert_threshold_pct INTEGER NOT NULL DEFAULT -1"))
+    conn.execute(text("ALTER TABLE frames ADD COLUMN battery_alert_sent INTEGER NOT NULL DEFAULT 0"))
+    Base.metadata.create_all(bind=conn)  # creates the two new tables only; existing ones untouched
+
+
 MIGRATIONS = [
     (1, _migration_1),
+    (2, _migration_2),
 ]
 
 
@@ -50,6 +62,7 @@ def run_migrations() -> None:
                 else:
                     conn.execute(text("UPDATE schema_version SET version = :v"), {"v": version})
     _ensure_frame_one()
+    _ensure_server_settings()
 
 
 def new_device_token() -> str:
@@ -144,3 +157,13 @@ def _ensure_frame_one() -> None:
             )
         else:
             logger.info("Fresh install: created default frame #%d", frame.id)
+
+
+def _ensure_server_settings() -> None:
+    """The SMTP config singleton (id=1) -- created with everything blank
+    (email sending disabled) the first time this runs; /admin edits it in
+    place from then on."""
+    with SessionLocal() as db:
+        if db.get(ServerSettings, 1) is None:
+            db.add(ServerSettings(id=1))
+            db.commit()