feat(threads): channel backend — discover (browse), public self-join, leave

The generic primitives channels need beyond dm/group, kept policy-governed and
scope-fenced (kernel never interprets 'channel'/'public'):

- discoverThreads(principal, filter): scope-wide browse (NOT membership-scoped)
  matching an opaque metadata filter, each result flagged joined; governed by
  iios.thread.discover (scope fence in the query). REST: GET /v1/threads/discover
- open self-join for PUBLIC channels: openThread now passes visibility into the
  join decision; dev-OPA iios.thread.join allows membership=channel+visibility=
  public (dm/group/private-channel stay invite-only)
- leaveThread + iios.thread.leave + REST DELETE /v1/threads/:id/me
- tests: public self-join vs private denied, discover joined-flags + group
  excluded + leave; dev-opa join-public/discover/leave rules (29 green)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/src/messaging/message.service.ts

@@ -114,10 +114,18 @@ export class MessageService {
     const actor = await this.actors.resolveActor(thread.scopeId, principal);
     const alreadyMember =
       (await this.prisma.iiosThreadParticipant.findUnique({ where: { threadId_actorId: { threadId, actorId: actor.id } } })) !== null;
-    // Join is governed ONLY for threads that opted into a membership model (chat dm/group);
-    // support/inbox/generic threads (no membership attr) keep open-join, unchanged.
-    const membership = (thread.metadata as { membership?: string } | null)?.membership;
-    await decideOrThrow(this.ports, { action: 'iios.thread.join', threadId, scopeId: thread.scopeId, membership, alreadyMember });
+    // Join is governed ONLY for threads that opted into a membership model (chat dm/group/channel);
+    // support/inbox/generic threads (no membership attr) keep open-join, unchanged. A PUBLIC channel
+    // is the one membership type that also allows open self-join (policy reads `visibility`).
+    const bag = (thread.metadata as { membership?: string; visibility?: string } | null) ?? {};
+    await decideOrThrow(this.ports, {
+      action: 'iios.thread.join',
+      threadId,
+      scopeId: thread.scopeId,
+      membership: bag.membership,
+      visibility: bag.visibility,
+      alreadyMember,
+    });
     await this.actors.ensureParticipant(threadId, actor.id);
     return { threadId, status: thread.status, history: await this.history(threadId) };
   }
@@ -228,6 +236,60 @@ export class MessageService {
     }));
   }
 
+  /**
+   * Scope-wide thread discovery — the ONE generic primitive channels need beyond dm/group.
+   * Unlike listThreads (membership-scoped), this returns threads in the caller's scope matching an
+   * opaque metadata filter (e.g. {membership:'channel', visibility:'public'}) REGARDLESS of whether
+   * the caller is a participant, each flagged `joined`. Governed by policy (fail-closed) + fenced to
+   * the caller's own scope. The kernel never interprets the filter keys.
+   */
+  async discoverThreads(
+    principal: MessagePrincipal,
+    filter?: { metadata?: Record<string, string> },
+  ): Promise<Array<{ threadId: string; subject: string | null; metadata: Record<string, unknown> | null; participantCount: number; joined: boolean }>> {
+    const scope = await this.actors.findScope(principal);
+    if (!scope) return [];
+    await decideOrThrow(this.ports, { action: 'iios.thread.discover', scopeId: scope.id });
+    const actor = await this.actors.resolveActor(scope.id, principal);
+
+    const inScope = await this.prisma.iiosThread.findMany({ where: { scopeId: scope.id } });
+    const metaFilter = filter?.metadata;
+    const matched = metaFilter
+      ? inScope.filter((t) => {
+          const bag = (t.metadata as Record<string, unknown> | null) ?? {};
+          return Object.entries(metaFilter).every(([k, v]) => bag[k] === v);
+        })
+      : inScope;
+    if (matched.length === 0) return [];
+
+    const ids = matched.map((t) => t.id);
+    const parts = await this.prisma.iiosThreadParticipant.groupBy({ by: ['threadId'], where: { threadId: { in: ids } }, _count: { actorId: true } });
+    const countBy = new Map(parts.map((p) => [p.threadId, p._count.actorId]));
+    const mine = await this.prisma.iiosThreadParticipant.findMany({ where: { threadId: { in: ids }, actorId: actor.id }, select: { threadId: true } });
+    const joinedSet = new Set(mine.map((m) => m.threadId));
+
+    return matched.map((t) => ({
+      threadId: t.id,
+      subject: t.subject,
+      metadata: (t.metadata as Record<string, unknown> | null) ?? null,
+      participantCount: countBy.get(t.id) ?? 0,
+      joined: joinedSet.has(t.id),
+    }));
+  }
+
+  /** Self-leave: remove the caller's own participant row. Governed (a member may always leave). */
+  async leaveThread(threadId: string, principal: MessagePrincipal): Promise<{ threadId: string; participantCount: number }> {
+    const thread = await this.prisma.iiosThread.findUnique({ where: { id: threadId } });
+    if (!thread) throw new NotFoundException('thread not found');
+    const actor = await this.actors.resolveActor(thread.scopeId, principal);
+    const callerP = await this.prisma.iiosThreadParticipant.findUnique({ where: { threadId_actorId: { threadId, actorId: actor.id } } });
+    const membership = (thread.metadata as { membership?: string } | null)?.membership;
+    await decideOrThrow(this.ports, { action: 'iios.thread.leave', threadId, scopeId: thread.scopeId, membership, callerRole: callerP?.participantRole });
+    await this.prisma.iiosThreadParticipant.deleteMany({ where: { threadId, actorId: actor.id } });
+    const participantCount = await this.prisma.iiosThreadParticipant.count({ where: { threadId } });
+    return { threadId, participantCount };
+  }
+
   /** Toggle the caller's per-thread notification mute flag. */
   async muteThread(threadId: string, principal: MessagePrincipal, muted: boolean): Promise<{ threadId: string; muted: boolean }> {
     const thread = await this.prisma.iiosThread.findUnique({ where: { id: threadId } });