feat(#382): Herald Service: broadcast to all active chat providers

- Replace direct DiscordService injection with CHAT_PROVIDERS array
- Herald broadcasts to ALL active chat providers (Discord, Matrix, future)
- Graceful error handling — one provider failure doesn't block others
- Skips disconnected providers automatically
- Tests verify multi-provider broadcasting behavior
- Fix lint: remove unnecessary conditional in matrix.service.ts

Refs #382

apps/api/src/bridge/matrix/matrix-room.service.ts

@@ -89,6 +89,21 @@ export class MatrixRoomService {
     return workspace?.matrixRoomId ?? null;
   }
 
+  /**
+   * Reverse lookup: find the workspace that owns a given Matrix room.
+   *
+   * @param roomId - The Matrix room ID (e.g. "!abc:example.com")
+   * @returns The workspace ID, or null if the room is not mapped to any workspace
+   */
+  async getWorkspaceForRoom(roomId: string): Promise<string | null> {
+    const workspace = await this.prisma.workspace.findFirst({
+      where: { matrixRoomId: roomId },
+      select: { id: true },
+    });
+
+    return workspace?.id ?? null;
+  }
+
   /**
    * Manually link an existing Matrix room to a workspace.
    *