fix: keep chat attachments while a linking chat exists

Fixes https://linear.app/codercom/issue/CODAGT-616/keep-chat-attachments-while-chats-remain-unarchived

Chat attachments could disappear even though the chat was still available. This happened when a message was saved without recording which attachments it used, or when cleanup deleted attachments before an archived chat itself was removed.

Creating a chat, sending or queuing a message, and editing a message now record both the message and which attachments it uses as one operation. If the chat is already at the 50-attachment limit, the chat change fails without being partially saved.

Concurrent attachment writes serialize the 50-file cap per chat. Cleanup locks candidates and checks again for new links before deleting. If a file becomes unavailable after input validation, create, send, and edit return a clear client error and roll back the chat change.

An attachment stays available while any chat that uses it still exists. After an archived chat reaches the end of its retention period and is deleted, an old attachment that no remaining chat uses can be cleaned up. The retention guide and unavailable-attachment UI text document this lifecycle. This change cannot restore attachments that were already deleted.

The database migration adds two indexes so attachment cleanup stays fast as attachments accumulate.

> This PR was authored by Mux (AI) on Mike's behalf.

coderd/database/dbmock/dbmock.go

@@ -1454,6 +1454,21 @@ func (mr *MockStoreMockRecorder) DeleteTask(ctx, arg any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "DeleteTask", reflect.TypeOf((*MockStore)(nil).DeleteTask), ctx, arg)
 }
 
+// DeleteUnlinkedChatFilesByIDs mocks base method.
+func (m *MockStore) DeleteUnlinkedChatFilesByIDs(ctx context.Context, arg database.DeleteUnlinkedChatFilesByIDsParams) (int64, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "DeleteUnlinkedChatFilesByIDs", ctx, arg)
+	ret0, _ := ret[0].(int64)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// DeleteUnlinkedChatFilesByIDs indicates an expected call of DeleteUnlinkedChatFilesByIDs.
+func (mr *MockStoreMockRecorder) DeleteUnlinkedChatFilesByIDs(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "DeleteUnlinkedChatFilesByIDs", reflect.TypeOf((*MockStore)(nil).DeleteUnlinkedChatFilesByIDs), ctx, arg)
+}
+
 // DeleteUserAIBudgetOverride mocks base method.
 func (m *MockStore) DeleteUserAIBudgetOverride(ctx context.Context, userID uuid.UUID) (database.UserAIBudgetOverride, error) {
 	m.ctrl.T.Helper()
@@ -4620,6 +4635,21 @@ func (mr *MockStoreMockRecorder) GetOAuth2ProviderAppsByUserID(ctx, userID any)
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetOAuth2ProviderAppsByUserID", reflect.TypeOf((*MockStore)(nil).GetOAuth2ProviderAppsByUserID), ctx, userID)
 }
 
+// GetOldUnlinkedChatFileIDs mocks base method.
+func (m *MockStore) GetOldUnlinkedChatFileIDs(ctx context.Context, arg database.GetOldUnlinkedChatFileIDsParams) ([]uuid.UUID, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetOldUnlinkedChatFileIDs", ctx, arg)
+	ret0, _ := ret[0].([]uuid.UUID)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetOldUnlinkedChatFileIDs indicates an expected call of GetOldUnlinkedChatFileIDs.
+func (mr *MockStoreMockRecorder) GetOldUnlinkedChatFileIDs(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetOldUnlinkedChatFileIDs", reflect.TypeOf((*MockStore)(nil).GetOldUnlinkedChatFileIDs), ctx, arg)
+}
+
 // GetOrganizationByID mocks base method.
 func (m *MockStore) GetOrganizationByID(ctx context.Context, id uuid.UUID) (database.Organization, error) {
 	m.ctrl.T.Helper()
@@ -8683,6 +8713,21 @@ func (mr *MockStoreMockRecorder) LinkChatFiles(ctx, arg any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "LinkChatFiles", reflect.TypeOf((*MockStore)(nil).LinkChatFiles), ctx, arg)
 }
 
+// LinkChatFilesAfterLock mocks base method.
+func (m *MockStore) LinkChatFilesAfterLock(ctx context.Context, arg database.LinkChatFilesAfterLockParams) (int32, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "LinkChatFilesAfterLock", ctx, arg)
+	ret0, _ := ret[0].(int32)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// LinkChatFilesAfterLock indicates an expected call of LinkChatFilesAfterLock.
+func (mr *MockStoreMockRecorder) LinkChatFilesAfterLock(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "LinkChatFilesAfterLock", reflect.TypeOf((*MockStore)(nil).LinkChatFilesAfterLock), ctx, arg)
+}
+
 // ListAIBridgeClients mocks base method.
 func (m *MockStore) ListAIBridgeClients(ctx context.Context, arg database.ListAIBridgeClientsParams) ([]string, error) {
 	m.ctrl.T.Helper()
@@ -9118,6 +9163,21 @@ func (mr *MockStoreMockRecorder) LockChatAndBumpSnapshotVersion(ctx, id any) *go
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "LockChatAndBumpSnapshotVersion", reflect.TypeOf((*MockStore)(nil).LockChatAndBumpSnapshotVersion), ctx, id)
 }
 
+// LockChatByID mocks base method.
+func (m *MockStore) LockChatByID(ctx context.Context, id uuid.UUID) (uuid.UUID, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "LockChatByID", ctx, id)
+	ret0, _ := ret[0].(uuid.UUID)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// LockChatByID indicates an expected call of LockChatByID.
+func (mr *MockStoreMockRecorder) LockChatByID(ctx, id any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "LockChatByID", reflect.TypeOf((*MockStore)(nil).LockChatByID), ctx, id)
+}
+
 // MarkAllInboxNotificationsAsRead mocks base method.
 func (m *MockStore) MarkAllInboxNotificationsAsRead(ctx context.Context, arg database.MarkAllInboxNotificationsAsReadParams) error {
 	m.ctrl.T.Helper()