Use ctx instead of db.DefaultContext in some packages(routers/services/modules) (#19163)

* Remove `db.DefaultContext` usage in routers, use `ctx` directly

* Use `ctx` directly if there is one, remove some `db.DefaultContext` in `services`

* Use ctx instead of db.DefaultContext for `cmd` and some `modules` packages

* fix incorrect context usage

services/mailer/mail.go

@@ -7,6 +7,7 @@ package mailer
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"html/template"
 	"mime"
@@ -414,6 +415,7 @@ func SendIssueAssignedMail(issue *models.Issue, doer *user_model.User, content s
 
 	for lang, tos := range langMap {
 		msgs, err := composeIssueCommentMessages(&mailCommentContext{
+			Context:    context.TODO(), // TODO: use a correct context
 			Issue:      issue,
 			Doer:       doer,
 			ActionType: models.ActionType(0),