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

web/src/core-bridge/generated/api.ts

@@ -1,5 +1,5 @@
 // Generated from backend DTOs; do not edit.
-// backend-contract-sha256: ba787115471d5d33fb0c33c95af014b5cd0d535455bd4bea037cfdcc4f048828
+// backend-contract-sha256: 54ea3f00fa672e0241efbb9fc8e7ad9e8fc68415ec124e08f70657c2d57d9ff9
 
 export interface AccountChannelBody {
   channel_id: string;
@@ -79,10 +79,10 @@ export interface ChannelInfo {
   expires_at: number;
   id: string;
   last_seq: number;
+  members_used: number;
   mode: ChannelMode;
   oldest_seq: number;
   password_required: boolean;
-  seats_used: number;
   settings: ChannelSettings;
 }
 export type ChannelMode = 'enc' | 'raw';
@@ -93,9 +93,9 @@ export interface ChannelSettings {
   file_max_downloads: number;
   join_policy: JoinPolicy;
   max_items: number;
+  max_members: number;
   members_can_upload: boolean;
   name?: string | null;
-  seats: number;
   ttl_seconds: number;
   ui_restrict_copy_download: boolean;
 }
@@ -106,9 +106,9 @@ export interface ChannelSettingsPatch {
   join_password?: string | null;
   join_policy?: JoinPolicy | null;
   max_items?: number | null;
+  max_members?: number | null;
   members_can_upload?: boolean | null;
   name?: string | null;
-  seats?: number | null;
   ttl_seconds?: number | null;
   ui_restrict_copy_download?: boolean | null;
 }
@@ -171,6 +171,10 @@ export interface GoneLogBody {
   error: string;
   oldest_seq: number;
 }
+export interface GuestInfo {
+  endpoint_hash: string;
+  joined_at: number;
+}
 export interface InvitationBody {
   channel_id: string;
   created_at: number;
@@ -198,11 +202,19 @@ export type JoinPolicy = 'open' | 'invite';
 export interface JoinRequest {
   password?: string | null;
 }
+export interface JoinResult {
+  joined_at: number;
+  members_max: number;
+  members_used: number;
+}
 export interface KeyPackageRequest {
   payload: string;
 }
 export interface MemberBody {
+  devices: number;
   joined_at: number;
+  online: boolean;
+  online_devices: number;
   role: string;
   username: string;
 }
@@ -260,18 +272,6 @@ export interface PublicKeyBody {
   public_key: string;
   signing_public_key?: string | null;
 }
-export interface SeatInfo {
-  account_name?: string | null;
-  endpoint_hash: string;
-  joined_at: number;
-  label?: string | null;
-}
-export interface SeatJoined {
-  endpoint_hash: string;
-  joined_at: number;
-  seats_max: number;
-  seats_used: number;
-}
 export interface ServerInfoBody {
   captcha: CaptchaBody;
   choices: SettingChoicesBody;
@@ -295,8 +295,8 @@ export interface LimitsBody {
   max_channel_bytes: number;
   max_frames_per_item: number;
   max_items: number;
+  max_members: number;
   max_payload_bytes: number;
-  max_seats: number;
   max_ttl_seconds: number;
   max_visitors: number;
   max_workspace_bytes: number;
@@ -305,8 +305,8 @@ export interface LimitsBody {
 }
 export interface SettingChoicesBody {
   file_max_downloads: number[];
+  max_members: number[];
   max_visitors: number[];
-  seats: number[];
   ttl_seconds: number[];
 }
 export interface ShareInfo {