Add per-room opt-in email notifications on mention (#67)

Lets a member subscribe to email when they're @mentioned in a room
while offline, alongside #66's always-on DM email. Debounced the same
way #66 is (one email per unread burst, not one per mention), and
deliberately scoped to regular rooms only -- DMs already have #66's
automatic offline email with no separate toggle needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas/room.py

@@ -63,12 +63,20 @@ class MyRoomItem(RoomRead):
     # avatar, not this room's internal `name`) without a second fetch per
     # row. None for a regular room.
     dm_partner: DmPartnerInfo | None = None
+    # #67: this viewer's own opt-in for "email me when mentioned here while
+    # offline" -- always false for a DM (see message_events.py, deliberately
+    # out of scope; DMs already get #66's automatic offline email).
+    email_notifications: bool = False
 
 
 class StartDmRequest(BaseModel):
     other_user_id: uuid.UUID
 
 
+class RoomNotificationSettingsUpdate(BaseModel):
+    email_notifications: bool
+
+
 class RoomMemberRead(BaseModel):
     user_id: uuid.UUID
     username: str