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.spec.ts

@@ -35,6 +35,7 @@ describe("MatrixRoomService", () => {
   const mockPrismaService = {
     workspace: {
       findUnique: vi.fn(),
+      findFirst: vi.fn(),
       update: vi.fn(),
     },
   };
@@ -162,6 +163,30 @@ describe("MatrixRoomService", () => {
     });
   });
 
+  describe("getWorkspaceForRoom", () => {
+    it("should return the workspace ID for a mapped room", async () => {
+      mockPrismaService.workspace.findFirst.mockResolvedValue({
+        id: "workspace-uuid-1",
+      });
+
+      const workspaceId = await service.getWorkspaceForRoom("!mapped-room:example.com");
+
+      expect(workspaceId).toBe("workspace-uuid-1");
+      expect(mockPrismaService.workspace.findFirst).toHaveBeenCalledWith({
+        where: { matrixRoomId: "!mapped-room:example.com" },
+        select: { id: true },
+      });
+    });
+
+    it("should return null for an unmapped room", async () => {
+      mockPrismaService.workspace.findFirst.mockResolvedValue(null);
+
+      const workspaceId = await service.getWorkspaceForRoom("!unknown-room:example.com");
+
+      expect(workspaceId).toBeNull();
+    });
+  });
+
   describe("linkWorkspaceToRoom", () => {
     it("should store the room mapping in the workspace", async () => {
       await service.linkWorkspaceToRoom("workspace-uuid-1", "!existing-room:example.com");