fix(presence): drop status='online' from registration insert

Mirrors the REST-login change in f17c46c on the registration insert path.
A successful POST /api/auth/register does not by itself imply a live
WebSocket — the client may never connect (transient network, mobile
background, error path between the 201 and /ws), leaving a permanently
stuck-online row that no disconnect timer can clean up. The schema
default 'offline' is correct; ws/handler.ts flips it to 'online' on real
WS auth.

The federated-stub upgrade path in the same handler is unaffected: it
only updates passwordHash/username/homeUserId/displayName/avatarColor,
leaving the stub's pre-existing 'offline' status (set when the stub was
created via replication) untouched.

Updates docs/systems/auth.md step 7 to reflect the new behavior.

docs/systems/auth.md

@@ -149,7 +149,7 @@ If `requestedAvatarColor` is provided and is in `AVATAR_COLORS`, use it. Otherwi
 4. Check username uniqueness (exact match on lowercased username)
 5. Hash password (bcrypt, 12 rounds)
 6. Generate Snowflake ID
-7. Insert user row with `status: 'online'`, admin flag if first user
+7. Insert user row (status defaults to `'offline'` at the schema level; it is set to `'online'` only when the client establishes a WebSocket via the WS auth path in `ws/handler.ts`). Admin flag set if first user.
 8. Sign JWT with `{ userId, username }`
 9. Return `{ token, user }` (user sanitized via `sanitizeUser(user, true)`)