[bugfix] Parse `scheduled_at` as ISO8601 with offset if RFC3339 parse fails (#4431)

# 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.

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4409 by reattempting `scheduled_at` parsing using ISO8601 offset.

## Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [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 not leveraged AI to create the proposed changes.
- [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.
- [x] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4431
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/internal/util/time.go b/internal/util/time.go
index 9ea8d0c93..7f75920e8 100644
--- a/internal/util/time.go
+++ b/internal/util/time.go
@@ -20,9 +20,10 @@ package util
 import "time"
 
 const (
-	ISO8601     = "2006-01-02T15:04:05.000Z"
-	ISO8601Date = "2006-01-02"
-	RFC2822     = "Mon, 02 Jan 2006 15:04:05 -0700"
+	ISO8601       = "2006-01-02T15:04:05.000Z"
+	ISO8601Offset = "2006-01-02T15:04:05-0700"
+	ISO8601Date   = "2006-01-02"
+	RFC2822       = "Mon, 02 Jan 2006 15:04:05 -0700"
 )
 
 // FormatISO8601 converts the given time to UTC and then formats it