[bugfix] Allow hyphen in usernameRelaxed regex (#4902)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Fix issue like the one described in https://codeberg.org/superseriousbusiness/gotosocial/issues/4878#issuecomment-17953256 where instance accounts containing hyphens can't send activities from their outbox.

## Checklist

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- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have not used so-called 'AI' to create the proposed changes.
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
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Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4902

internal/regexes/regexes.go

@@ -50,7 +50,7 @@ const (
 	emojiValidator           = `^` + emojiShortcode + `$`                                // Validate a single emoji shortcode.
 	doubleSpaceFinder        = `(?:\b)?(?:\s|^) (?:\b)?`                                 // Extract all double whitespaces from a text.
 	usernameStrict           = `^[a-z0-9_]{1,64}$`                                       // Pattern for usernames on THIS instance. maximumUsernameLength = 64
-	usernameRelaxed          = `[a-z0-9_\.]{1,}`                                         // Relaxed version of username that can match instance accounts too.
+	usernameRelaxed          = `[a-z0-9_\-\.]{1,}`                                       // Relaxed version of username that can match instance accounts too (includes characters commonly found in domain names).
 	misskeyReportNotesFinder = `(?m)(?:^Note: ((?:http|https):\/\/.*)$)`                 // Extract reported Note URIs from the text of a Misskey report/flag.
 	ulid                     = `[0123456789ABCDEFGHJKMNPQRSTVWXYZ]{26}`                  // Pattern for ULID.
 	ulidValidate             = `^` + ulid + `$`                                          // Validate one ULID.