feat(api)!: fold devices into users and report member presence

Contract 7.0.0 (breaking). Devices become an attribute of the account;
members are managed per user, and only unauthenticated guests keep a
per-terminal registration.

- rename seats -> max_members, seats_used -> members_used,
  seats_full -> members_full, SeatJoined -> JoinResult, the `seats`
  stream control message -> `members`, and POST /channels/{id}/seats ->
  POST /channels/{id}/join (stored channel settings are migrated on start)
- drop per-terminal encrypted device labels and the seat list/release
  endpoints; add GET/DELETE /channels/{id}/guests for guest terminals
- authenticate account members by account session only: the terminal key
  is optional for logged-in accounts, no seat row is written for them,
  and gated-download grants are counted per user (per terminal for guests)
- GET /channels/{id}/members now returns online, devices and
  online_devices per account; presence is tracked in memory from active
  WebSocket/SSE subscriptions and is not pushed over the stream
- owner invitation list no longer returns accounts that already joined

server/scripts/smoke.ps1

@@ -46,12 +46,12 @@ try {
     $ownerH = Headers 'owner' $ownerTok $ownerSecret
 
     $created = Invoke-RestMethod "$base/api/v1/channels" -Method Post -Body $body -ContentType 'application/json' -Headers $ownerH
-    Check ($created.id -eq (B64 $id) -and $created.seats_used -eq 1) '创建频道成功'
+    Check ($created.id -eq (B64 $id) -and $created.members_used -eq 1) '创建频道成功'
     $cid = B64 $id
 
     $memberH = Headers 'write' $writeTok $memberSecret
-    $joined = Invoke-RestMethod "$base/api/v1/channels/$cid/seats" -Method Post -Headers $memberH
-    Check ($joined.seats_used -eq 2) '成员加入'
+    $joined = Invoke-RestMethod "$base/api/v1/channels/$cid/join" -Method Post -Headers $memberH
+    Check ($joined.members_used -eq 2) '成员加入'
 
     # enc 帧:负载 = 1024 档位 + 16 标签
     $item = Rand 16

server/scripts/smoke.py

@@ -71,8 +71,8 @@ try:
     check(status == 201, "创建频道")
     ch = b64(cid)
 
-    status, body = call("POST", f"/api/v1/channels/{ch}/seats", member_h)
-    check(status == 200 and json.loads(body)["seats_used"] == 2, "成员加入")
+    status, body = call("POST", f"/api/v1/channels/{ch}/join", member_h)
+    check(status == 200 and json.loads(body)["members_used"] == 2, "成员加入")
 
     item = os.urandom(16)
     f = frame(item, 0, 9, 1040, 0x42)