[feature] Conversations API (#3013)

* Implement conversations API

* Sort and page conversations by last status ID

* Appease linter

* Fix deleting conversations and statuses

* Refactor to make migrations automatic

* Lint

* Update tests post-merge

* Fixes from live-fire testing

* Linter caught a format problem

* Refactor tests, fix cache

* Negative test for non-DMs

* Run conversations advanced migration on testrig startup as well as regular server startup

* Document (lack of) side effects of API method for deleting a conversation

* Make not-found check less nested for readability

* Rename PutConversation to UpsertConversation

* Use util.Ptr instead of IIFE

* Reduce cache used by conversations

* Remove unnecessary TableExpr/ColumnExpr

* Use struct tags for both unique constraints on Conversation

* Make it clear how paging with GetDirectStatusIDsBatch should be used

* Let conversation paging skip conversations it can't render

* Use Bun NewDropTable

* Convert delete raw query to Bun

* Convert update raw query to Bun

* Convert latestConversationStatusesTempTable raw query partially to Bun

* Convert conversationStatusesTempTable raw query partially to Bun

* Rename field used to store result of MaxDirectStatusID

* Move advanced migrations to their own tiny processor

* Catch up util function name with main

* Remove json.… wrappers

* Remove redundant check

* Combine error checks

* Replace map with slice of structs

* Address processor/type converter comments

- Add context info for errors
- Extract some common processor code into shared methods
- Move conversation eligibility check ahead of populating conversation

* Add error context when dropping temp tables

internal/db/bundb/status.go

@@ -682,3 +682,35 @@ func (s *statusDB) getStatusBoostIDs(ctx context.Context, statusID string) ([]st
 		return statusIDs, nil
 	})
 }
+
+func (s *statusDB) MaxDirectStatusID(ctx context.Context) (string, error) {
+	maxID := ""
+	if err := s.db.
+		NewSelect().
+		Model((*gtsmodel.Status)(nil)).
+		ColumnExpr("COALESCE(MAX(?), '')", bun.Ident("id")).
+		Where("? = ?", bun.Ident("visibility"), gtsmodel.VisibilityDirect).
+		Scan(ctx, &maxID); // nocollapse
+	err != nil {
+		return "", err
+	}
+	return maxID, nil
+}
+
+func (s *statusDB) GetDirectStatusIDsBatch(ctx context.Context, minID string, maxIDInclusive string, count int) ([]string, error) {
+	var statusIDs []string
+	if err := s.db.
+		NewSelect().
+		Model((*gtsmodel.Status)(nil)).
+		Column("id").
+		Where("? = ?", bun.Ident("visibility"), gtsmodel.VisibilityDirect).
+		Where("? > ?", bun.Ident("id"), minID).
+		Where("? <= ?", bun.Ident("id"), maxIDInclusive).
+		Order("id ASC").
+		Limit(count).
+		Scan(ctx, &statusIDs); // nocollapse
+	err != nil {
+		return nil, err
+	}
+	return statusIDs, nil
+}