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/app/schemas/site_invite.py

@@ -0,0 +1,31 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, EmailStr, Field
+
+from app.models import InviteStatus
+
+
+class SiteInviteCreate(BaseModel):
+    email: EmailStr
+
+
+class SiteInviteRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    email: str
+    invited_by: uuid.UUID
+    status: InviteStatus
+    expires_at: datetime
+    created_at: datetime
+
+
+class SignupValidateRead(BaseModel):
+    email: str
+
+
+class SignupComplete(BaseModel):
+    token: str
+    username: str = Field(min_length=3, max_length=50)
+    password: str = Field(min_length=8, max_length=200)