[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/typeutils/internaltofrontend.go

@@ -1739,6 +1739,67 @@ func (c *Converter) NotificationToAPINotification(
 	}, nil
 }
 
+// ConversationToAPIConversation converts a conversation into its API representation.
+// The conversation status will be filtered using the notification filter context,
+// and may be nil if the status was hidden.
+func (c *Converter) ConversationToAPIConversation(
+	ctx context.Context,
+	conversation *gtsmodel.Conversation,
+	requestingAccount *gtsmodel.Account,
+	filters []*gtsmodel.Filter,
+	mutes *usermute.CompiledUserMuteList,
+) (*apimodel.Conversation, error) {
+	apiConversation := &apimodel.Conversation{
+		ID:     conversation.ID,
+		Unread: !*conversation.Read,
+	}
+	for _, account := range conversation.OtherAccounts {
+		var apiAccount *apimodel.Account
+		blocked, err := c.state.DB.IsEitherBlocked(ctx, requestingAccount.ID, account.ID)
+		if err != nil {
+			return nil, gtserror.Newf(
+				"DB error checking blocks between accounts %s and %s: %w",
+				requestingAccount.ID,
+				account.ID,
+				err,
+			)
+		}
+		if blocked || account.IsSuspended() {
+			apiAccount, err = c.AccountToAPIAccountBlocked(ctx, account)
+		} else {
+			apiAccount, err = c.AccountToAPIAccountPublic(ctx, account)
+		}
+		if err != nil {
+			return nil, gtserror.Newf(
+				"error converting account %s to API representation: %w",
+				account.ID,
+				err,
+			)
+		}
+		apiConversation.Accounts = append(apiConversation.Accounts, *apiAccount)
+	}
+	if conversation.LastStatus != nil {
+		var err error
+		apiConversation.LastStatus, err = c.StatusToAPIStatus(
+			ctx,
+			conversation.LastStatus,
+			requestingAccount,
+			statusfilter.FilterContextNotifications,
+			filters,
+			mutes,
+		)
+		if err != nil && !errors.Is(err, statusfilter.ErrHideStatus) {
+			return nil, gtserror.Newf(
+				"error converting status %s to API representation: %w",
+				conversation.LastStatus.ID,
+				err,
+			)
+		}
+	}
+
+	return apiConversation, nil
+}
+
 // DomainPermToAPIDomainPerm converts a gts model domin block or allow into an api domain permission.
 func (c *Converter) DomainPermToAPIDomainPerm(
 	ctx context.Context,