fix(server): local-fast-path for invite snapshot — skip HTTP self-reach

POST /api/dm/space-invite was hanging 5s and returning invite_invalid
for any local-space invite. fetchSpaceInviteSnapshot was being called
against our own public domain from inside the backspace container, which
fails (Docker NAT loopback) and aborts on timeout.

Add getLocalInviteSnapshot — reads the snapshot directly from the DB —
and branch in dm.ts so local invites bypass the HTTP roundtrip entirely.
Cross-instance invites still go through fetchSpaceInviteSnapshot with
its existing SSRF guard.

Also refactor the GET /api/spaces/invite/:code/preview handler to use
the same helper, keeping the snapshot shape in one place.

Tests assert fetchSpaceInviteSnapshot is NOT called for the local case
(critical regression guard) and that the cross-instance path still hits
the HTTP fetch.

packages/server/src/routes/dm.spaceInvite.test.ts

@@ -59,11 +59,13 @@ vi.mock('../utils/federationAuth.js', async (importActual) => {
   return { ...actual, getOurOrigin: () => 'https://local.test' };
 });
 
-// Mock the snapshot fetch so tests don't make real HTTP calls.
+// Mock the snapshot helpers so tests don't make real HTTP calls and we can
+// observe which lookup path the route takes (local DB vs. cross-instance HTTP).
 vi.mock('../utils/spaceInviteSnapshot.js', () => ({
   fetchSpaceInviteSnapshot: vi.fn(),
+  getLocalInviteSnapshot: vi.fn(),
 }));
-import { fetchSpaceInviteSnapshot } from '../utils/spaceInviteSnapshot.js';
+import { fetchSpaceInviteSnapshot, getLocalInviteSnapshot } from '../utils/spaceInviteSnapshot.js';
 
 function applyMigrations(db: Database.Database): void {
   const migrationsDir = path.resolve(__dirname, '../../drizzle');
@@ -130,6 +132,7 @@ describe('POST /api/dm/space-invite', () => {
     seedFriendship('alice', 'bob');
     currentUserId = 'alice';
     (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReset();
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReset();
     app = await buildApp();
   });
 
@@ -147,12 +150,13 @@ describe('POST /api/dm/space-invite', () => {
     });
     expect(res.statusCode).toBe(400);
     expect(JSON.parse(res.body).error).toBe('not_a_friend');
-    // Snapshot should not be fetched if friendship gate fails first.
+    // Snapshot should not be looked up if friendship gate fails first.
     expect(fetchSpaceInviteSnapshot).not.toHaveBeenCalled();
+    expect(getLocalInviteSnapshot).not.toHaveBeenCalled();
   });
 
-  it('rejects 400 invite_invalid when upstream preview 404s (snapshot null)', async () => {
-    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce(null);
+  it('rejects 400 invite_invalid when local snapshot lookup returns null', async () => {
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReturnValueOnce(null);
     const res = await app.inject({
       method: 'POST',
       url: '/api/dm/space-invite',
@@ -171,7 +175,7 @@ describe('POST /api/dm/space-invite', () => {
   });
 
   it('rejects 400 invite_invalid when snapshot.spaceId mismatches the requested spaceId', async () => {
-    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReturnValueOnce({
       spaceId: 'WRONG',
       spaceName: 'X',
       description: null,
@@ -197,7 +201,7 @@ describe('POST /api/dm/space-invite', () => {
   });
 
   it('inserts a type=system message with parseable space_invite content on success', async () => {
-    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReturnValueOnce({
       spaceId: 'S1',
       spaceName: 'Aether',
       description: 'desc',
@@ -247,7 +251,7 @@ describe('POST /api/dm/space-invite', () => {
   });
 
   it('reuses an existing 1-on-1 DM rather than creating a new one', async () => {
-    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValue({
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReturnValue({
       spaceId: 'S1',
       spaceName: 'Aether',
       description: null,
@@ -297,4 +301,89 @@ describe('POST /api/dm/space-invite', () => {
     expect(messages.length).toBe(2);
     expect(messages.every(m => m.type === 'system')).toBe(true);
   });
+
+  it('uses local DB lookup (skips HTTP) when spaceInstanceOrigin is empty/local', async () => {
+    // Regression guard for the production hang: when the space is local, the
+    // route MUST NOT call fetchSpaceInviteSnapshot — that path tries to reach
+    // our own public domain over HTTPS, which fails inside Docker (NAT loopback).
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReturnValueOnce({
+      spaceId: 'S1',
+      spaceName: 'Aether',
+      description: null,
+      icon: null,
+      avatarColor: null,
+      memberCount: 1,
+      instanceName: 'Backspace',
+    });
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: '',
+        inviteCode: 'abc',
+      },
+    });
+    expect(res.statusCode).toBe(200);
+    expect(getLocalInviteSnapshot).toHaveBeenCalledWith('abc');
+    // CRITICAL: the HTTP fetch path must NOT run for local invites.
+    expect(fetchSpaceInviteSnapshot).not.toHaveBeenCalled();
+  });
+
+  it('uses local DB lookup when spaceInstanceOrigin equals our own origin', async () => {
+    // Same fast-path applies if the client sends our origin explicitly.
+    (getLocalInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockReturnValueOnce({
+      spaceId: 'S1',
+      spaceName: 'Aether',
+      description: null,
+      icon: null,
+      avatarColor: null,
+      memberCount: 1,
+      instanceName: 'Backspace',
+    });
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: 'https://local.test',
+        inviteCode: 'abc',
+      },
+    });
+    expect(res.statusCode).toBe(200);
+    expect(getLocalInviteSnapshot).toHaveBeenCalledWith('abc');
+    expect(fetchSpaceInviteSnapshot).not.toHaveBeenCalled();
+  });
+
+  it('uses HTTP fetch path for cross-instance invites', async () => {
+    // Regression guard: when the space is on a different instance, we must
+    // hit the SSRF-validated HTTP path, not the local DB.
+    (fetchSpaceInviteSnapshot as unknown as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+      spaceId: 'S1',
+      spaceName: 'Remote',
+      description: null,
+      icon: null,
+      avatarColor: null,
+      memberCount: 5,
+      instanceName: 'OtherHost',
+    });
+
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/dm/space-invite',
+      payload: {
+        target: { userId: 'bob' },
+        spaceId: 'S1',
+        spaceInstanceOrigin: 'https://remote.example',
+        inviteCode: 'abc',
+      },
+    });
+    expect(res.statusCode).toBe(200);
+    expect(fetchSpaceInviteSnapshot).toHaveBeenCalledWith('https://remote.example', 'abc');
+    expect(getLocalInviteSnapshot).not.toHaveBeenCalled();
+  });
 });