Add resizable room panel, searchable user picker, and direct room membership

Room info panel is now user-resizable (fixing a layout clip at narrow
widths), and every user-selection spot (room membership, admin ownership
transfer) uses a new searchable UserPicker instead of raw text input or
prompt(). Member rows fold role + actions into a single inline dropdown
instead of a row of buttons, so the member list stays usable as rooms grow.

Room invites (the accept/decline flow) are replaced by adding a user to a
room directly -- an admin/owner picks someone and they're a member
immediately, with a "you've been added" notification email instead of an
invite email. Drops the now-unused room_invites table.

backend/app/models/__init__.py

@@ -2,7 +2,7 @@ from app.models.admin_audit_log import AdminAuditLog
 from app.models.api_token import ApiToken
 from app.models.base import Base
 from app.models.event_subscription import EventSubscription
-from app.models.invite import InviteStatus, RoomInvite
+from app.models.invite import InviteStatus
 from app.models.membership import RoomMembership, RoomRole
 from app.models.message import Message
 from app.models.message_image import MessageImage
@@ -23,7 +23,6 @@ __all__ = [
     "Message",
     "MessageImage",
     "MessageReaction",
-    "RoomInvite",
     "InviteStatus",
     "SiteInvite",
     "SmtpSettings",