feat: auto-archive inactive chats with audit trail (#24642)

Adds a background job in `dbpurge` that periodically archives chats
inactive beyond a configurable threshold. Each archived root chat gets a
background audit entry tagged `chat_auto_archive`. Disabled by default.

* New `AutoArchiveInactiveChats` SQL query with LATERAL last-activity
subquery and partial index on archive candidates
* `site_configs`-backed `auto_archive_days` setting with admin-only PUT,
any-authenticated-user GET
* Cascade archive via `root_chat_id`; pinned chats and active threads
exempt
* Root-only audit dispatch on detached context, matching manual archive
(`patchChat`) behavior
* 11 subtests covering disabled no-op, boundary, deleted messages, child
activity, pinned exemption, multi-owner, idempotency, and batch
pagination

PR #24643 adds per-owner digest notifications.
PR #24704 adds the requisite UI controls.

> 🤖

coderd/database/dbauthz/dbauthz.go

@@ -644,6 +644,8 @@ var (
 					rbac.ResourceNotificationMessage.Type:  {policy.ActionDelete},
 					rbac.ResourceApiKey.Type:               {policy.ActionDelete},
 					rbac.ResourceAibridgeInterception.Type: {policy.ActionDelete},
+					// Chat auto-archive sets archived=true on inactive chats.
+					rbac.ResourceChat.Type: {policy.ActionRead, policy.ActionUpdate},
 				}),
 				User:    []rbac.Permission{},
 				ByOrgID: map[string]rbac.OrgPermissions{},
@@ -1593,6 +1595,16 @@ func (q *querier) ArchiveUnusedTemplateVersions(ctx context.Context, arg databas
 	return q.db.ArchiveUnusedTemplateVersions(ctx, arg)
 }
 
+func (q *querier) AutoArchiveInactiveChats(ctx context.Context, arg database.AutoArchiveInactiveChatsParams) ([]database.AutoArchiveInactiveChatsRow, error) {
+	// Background write by dbpurge. The LATERAL read of chat_messages rows
+	// happens below the RBAC boundary; only the chat row itself requires
+	// authorization.
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceChat); err != nil {
+		return nil, err
+	}
+	return q.db.AutoArchiveInactiveChats(ctx, arg)
+}
+
 func (q *querier) BackoffChatDiffStatus(ctx context.Context, arg database.BackoffChatDiffStatusParams) error {
 	// This is a system-level operation used by the gitsync
 	// background worker to reschedule failed refreshes. Same
@@ -2557,6 +2569,17 @@ func (q *querier) GetAuthorizationUserRoles(ctx context.Context, userID uuid.UUI
 	return q.db.GetAuthorizationUserRoles(ctx, userID)
 }
 
+func (q *querier) GetChatAutoArchiveDays(ctx context.Context, defaultAutoArchiveDays int32) (int32, error) {
+	// Chat auto-archive is a deployment-wide config read by dbpurge.
+	// Only requires a valid actor in context. The HTTP GET handler
+	// allows any authenticated user; the PUT handler enforces admin
+	// access (policy.ActionUpdate on ResourceDeploymentConfig).
+	if _, ok := ActorFromContext(ctx); !ok {
+		return 0, ErrNoActor
+	}
+	return q.db.GetChatAutoArchiveDays(ctx, defaultAutoArchiveDays)
+}
+
 func (q *querier) GetChatByID(ctx context.Context, id uuid.UUID) (database.Chat, error) {
 	return fetch(q.log, q.auth, q.db.GetChatByID)(ctx, id)
 }
@@ -7374,6 +7397,13 @@ func (q *querier) UpsertBoundaryUsageStats(ctx context.Context, arg database.Ups
 	return q.db.UpsertBoundaryUsageStats(ctx, arg)
 }
 
+func (q *querier) UpsertChatAutoArchiveDays(ctx context.Context, autoArchiveDays int32) error {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return err
+	}
+	return q.db.UpsertChatAutoArchiveDays(ctx, autoArchiveDays)
+}
+
 func (q *querier) UpsertChatDebugLoggingAllowUsers(ctx context.Context, allowUsers bool) error {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return err

coderd/database/dbauthz/dbauthz_test.go

@@ -741,6 +741,18 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().UpsertChatRetentionDays(gomock.Any(), int32(30)).Return(nil).AnyTimes()
 		check.Args(int32(30)).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
 	}))
+	s.Run("GetChatAutoArchiveDays", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatAutoArchiveDays(gomock.Any(), gomock.Any()).Return(int32(90), nil).AnyTimes()
+		check.Args(int32(90)).Asserts()
+	}))
+	s.Run("UpsertChatAutoArchiveDays", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().UpsertChatAutoArchiveDays(gomock.Any(), int32(90)).Return(nil).AnyTimes()
+		check.Args(int32(90)).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
+	}))
+	s.Run("AutoArchiveInactiveChats", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().AutoArchiveInactiveChats(gomock.Any(), database.AutoArchiveInactiveChatsParams{}).Return([]database.AutoArchiveInactiveChatsRow{}, nil).AnyTimes()
+		check.Args(database.AutoArchiveInactiveChatsParams{}).Asserts(rbac.ResourceChat, policy.ActionUpdate)
+	}))
 	s.Run("GetChatMessageByID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
 		msg := testutil.Fake(s.T(), faker, database.ChatMessage{ChatID: chat.ID})