[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

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

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>

internal/api/client/statuses/statuscreate_test.go

@@ -467,7 +467,7 @@ func (suite *StatusCreateTestSuite) TestPostNewStatusMessedUpIntPolicy() {
 }`, out)
 }
 
-func (suite *StatusCreateTestSuite) TestPostNewScheduledStatus() {
+func (suite *StatusCreateTestSuite) TestPostNewScheduledStatusRFC3339() {
 	out, recorder := suite.postStatus(map[string][]string{
 		"status":       {"this is a brand new status! #helloworld"},
 		"spoiler_text": {"hello hello"},
@@ -497,6 +497,56 @@ func (suite *StatusCreateTestSuite) TestPostNewScheduledStatus() {
 }`, out)
 }
 
+func (suite *StatusCreateTestSuite) TestPostNewScheduledStatusISO8601Offset() {
+	out, recorder := suite.postStatus(map[string][]string{
+		"status":       {"this is a brand new status! #helloworld"},
+		"spoiler_text": {"hello hello"},
+		"sensitive":    {"true"},
+		"visibility":   {string(apimodel.VisibilityMutualsOnly)},
+		"scheduled_at": {"2080-09-05T19:38:00+0400"},
+	}, "")
+
+	// We should have OK from
+	// our call to the function.
+	suite.Equal(http.StatusOK, recorder.Code)
+
+	// A scheduled status with scheduled_at and status params should be returned.
+	suite.Equal(`{
+  "id": "ZZZZZZZZZZZZZZZZZZZZZZZZZZ",
+  "media_attachments": [],
+  "params": {
+    "application_id": "01F8MGY43H3N2C8EWPR2FPYEXG",
+    "language": "",
+    "scheduled_at": null,
+    "sensitive": true,
+    "spoiler_text": "hello hello",
+    "text": "this is a brand new status! #helloworld",
+    "visibility": "private"
+  },
+  "scheduled_at": "2080-09-05T15:38:00.000Z"
+}`, out)
+}
+
+func (suite *StatusCreateTestSuite) TestPostNewScheduledStatusNonsenseTime() {
+	out, recorder := suite.postStatus(map[string][]string{
+		"status":       {"this is a brand new status! #helloworld"},
+		"spoiler_text": {"hello hello"},
+		"sensitive":    {"true"},
+		"visibility":   {string(apimodel.VisibilityMutualsOnly)},
+		"scheduled_at": {"pee pee poo poo"},
+	}, "")
+
+	// We should have 400 from
+	// our call to the function.
+	suite.Equal(http.StatusBadRequest, recorder.Code)
+
+	// We should have a helpful error
+	// message telling us how we screwed up.
+	suite.Equal(`{
+  "error": "Bad Request: could not parse scheduled_at value pee pee poo poo as ISO8601 time"
+}`, out)
+}
+
 func (suite *StatusCreateTestSuite) TestPostNewBackfilledStatus() {
 	// A time in the past.
 	scheduledAtStr := "2020-10-04T15:32:02.018Z"