Add admin-invited signups and email notifications (Gitea issue #15)

Site admins can invite a brand-new person by email from the Admin portal
Users tab -- a signup-link email lets them set their own username/password
and lands them in the app already logged in. Existing users invited to a
room now also get an email. Closes the "invited but never notified" gap
from both directions.

SMTP is configured through the Admin Settings tab at runtime (not the env
file), persisted in a new smtp_settings table with the password encrypted
at rest via a Fernet key derived from SESSION_SECRET -- the first
reversible secret this app stores in the database. A "send test email"
button surfaces real delivery errors; the invite/notification paths
themselves never fail loudly, since an SMTP outage shouldn't block an
action that already succeeded in the database.

New site_invites table mirrors RoomInvite's shape but targets an email
address with no room context; the raw signup token is hashed the same way
API tokens are, and only ever exists in the email link. POST /api/signup
is the first genuinely public, unauthenticated account-creation endpoint
in this app, reusing the existing register_user path for identical
validation.

backend/tests/test_invites.py

@@ -1,10 +1,28 @@
 import uuid
 from datetime import datetime, timedelta, timezone
 
-from app.models import RoomInvite
+from app.models import RoomInvite, User
 from tests.conftest import login_as, register_and_login
 
 
+async def _make_admin(db_session, user_id: str) -> None:
+    user = await db_session.get(User, uuid.UUID(user_id))
+    user.is_site_admin = True
+    await db_session.commit()
+
+
+async def _configure_smtp(client):
+    resp = await client.put(
+        "/api/admin/settings/smtp",
+        json={
+            "host": "smtp.example.com",
+            "port": 587,
+            "from_address": "noreply@example.com",
+        },
+    )
+    assert resp.status_code == 200, resp.text
+
+
 async def _create_private_room(client, name="secret"):
     resp = await client.post("/api/rooms", json={"name": name, "is_private": True})
     assert resp.status_code == 201, resp.text
@@ -171,3 +189,45 @@ async def test_expired_invite_rejected_on_accept(client, db_session):
     await login_as(client, "bob")
     resp = await client.post(f"/api/invites/{invite['id']}/accept")
     assert resp.status_code == 400
+
+
+async def test_create_invite_sends_email_to_target(client, db_session, monkeypatch):
+    calls = []
+
+    async def fake_send(message, **kwargs):
+        calls.append(kwargs)
+
+    monkeypatch.setattr("app.services.email_service.aiosmtplib.send", fake_send)
+
+    alice = await register_and_login(client, db_session, username="alice")
+    await _make_admin(db_session, alice["id"])
+    await _configure_smtp(client)
+    room = await _create_private_room(client)
+    await register_and_login(client, db_session, username="bob")
+    await login_as(client, "alice")
+
+    resp = await client.post(
+        f"/api/rooms/{room['id']}/invites", json={"target_username": "bob"}
+    )
+    assert resp.status_code == 201
+    assert len(calls) == 1
+    assert calls[0]["hostname"] == "smtp.example.com"
+
+
+async def test_create_invite_succeeds_even_if_email_delivery_fails(client, db_session, monkeypatch):
+    async def fake_send(message, **kwargs):
+        raise ConnectionRefusedError("boom")
+
+    monkeypatch.setattr("app.services.email_service.aiosmtplib.send", fake_send)
+
+    alice = await register_and_login(client, db_session, username="alice")
+    await _make_admin(db_session, alice["id"])
+    await _configure_smtp(client)
+    room = await _create_private_room(client)
+    await register_and_login(client, db_session, username="bob")
+    await login_as(client, "alice")
+
+    resp = await client.post(
+        f"/api/rooms/{room['id']}/invites", json={"target_username": "bob"}
+    )
+    assert resp.status_code == 201