fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

models/issues/issue_update.go

@@ -263,14 +263,46 @@ func AddDeletePRBranchComment(ctx context.Context, doer *user_model.User, repo *
 	return err
 }
 
+// validateAttachmentForIssue rejects a foreign or already-linked attachment before it is linked to
+// issue: a known UUID could otherwise re-link (and expose) another repo's private attachment. A
+// legacy attachment predating repo_id-on-upload is adopted into the issue's repo.
+func validateAttachmentForIssue(ctx context.Context, issue *Issue, attachment *repo_model.Attachment) error {
+	if attachment.RepoID == 0 && attachment.CreatedUnix < repo_model.LegacyAttachmentMissingRepoIDCutoff {
+		attachment.RepoID = issue.RepoID
+		if err := repo_model.UpdateAttachmentByUUID(ctx, attachment, "repo_id"); err != nil {
+			return fmt.Errorf("update attachment repo_id [id: %d]: %w", attachment.ID, err)
+		}
+	}
+	if attachment.RepoID != issue.RepoID {
+		return util.NewPermissionDeniedErrorf("attachment belongs to a different repository")
+	}
+	if attachment.IssueID != 0 && attachment.IssueID != issue.ID {
+		return util.NewPermissionDeniedErrorf("attachment is already linked to another issue")
+	}
+	if attachment.ReleaseID != 0 {
+		return util.NewPermissionDeniedErrorf("attachment is already linked to a release")
+	}
+	return nil
+}
+
 // UpdateIssueAttachments update attachments by UUIDs for the issue
 func UpdateIssueAttachments(ctx context.Context, issueID int64, uuids []string) (err error) {
+	if len(uuids) == 0 {
+		return nil
+	}
 	return db.WithTx(ctx, func(ctx context.Context) error {
+		issue, err := GetIssueByID(ctx, issueID)
+		if err != nil {
+			return err
+		}
 		attachments, err := repo_model.GetAttachmentsByUUIDs(ctx, uuids)
 		if err != nil {
 			return fmt.Errorf("getAttachmentsByUUIDs [uuids: %v]: %w", uuids, err)
 		}
 		for i := range attachments {
+			if err := validateAttachmentForIssue(ctx, issue, attachments[i]); err != nil {
+				return err
+			}
 			attachments[i].IssueID = issueID
 			if err := repo_model.UpdateAttachment(ctx, attachments[i]); err != nil {
 				return fmt.Errorf("update attachment [id: %d]: %w", attachments[i].ID, err)