[feature] scheduled statuses (#4274)

An implementation of [`scheduled_statuses`](https://docs.joinmastodon.org/methods/scheduled_statuses/). Will fix #1006.

this is heavily WIP and I need to reorganize some of the code, working on this made me somehow familiar with the codebase and led to my other recent contributions
i told some fops on fedi i'd work on this so i have no choice but to complete it 🤷‍♀️
btw iirc my avatar presents me working on this branch

Signed-off-by: nicole mikołajczyk <git@mkljczk.pl>
Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4274
Co-authored-by: nicole mikołajczyk <git@mkljczk.pl>
Co-committed-by: nicole mikołajczyk <git@mkljczk.pl>

internal/api/client/scheduledstatuses/scheduledstatusesget.go

@@ -0,0 +1,136 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package scheduledstatuses
+
+import (
+	"net/http"
+
+	apiutil "code.superseriousbusiness.org/gotosocial/internal/api/util"
+	"code.superseriousbusiness.org/gotosocial/internal/gtserror"
+	"code.superseriousbusiness.org/gotosocial/internal/paging"
+	"github.com/gin-gonic/gin"
+)
+
+// ScheduledStatusesGETHandler swagger:operation GET /api/v1/scheduled_statuses getScheduledStatuses
+//
+// Get an array of statuses scheduled by authorized user.
+//
+//	---
+//	tags:
+//	- scheduled_statuses
+//
+//	produces:
+//	- application/json
+//
+//	parameters:
+//	-
+//		name: max_id
+//		type: string
+//		description: >-
+//			Return only statuses *OLDER* than the given max status ID.
+//			The status with the specified ID will not be included in the response.
+//		in: query
+//		required: false
+//	-
+//		name: since_id
+//		type: string
+//		description: >-
+//			Return only statuses *newer* than the given since status ID.
+//			The status with the specified ID will not be included in the response.
+//		in: query
+//	-
+//		name: min_id
+//		type: string
+//		description: >-
+//			Return only statuses *immediately newer* than the given min ID.
+//			The status with the specified ID will not be included in the response.
+//		in: query
+//		required: false
+//	-
+//		name: limit
+//		type: integer
+//		description: Number of scheduled statuses to return.
+//		default: 20
+//		in: query
+//		required: false
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- read:statuses
+//
+//	responses:
+//		'200':
+//			headers:
+//				Link:
+//					type: string
+//					description: Links to the next and previous queries.
+//			schema:
+//				type: array
+//				items:
+//					"$ref": "#/definitions/scheduledStatus"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'404':
+//			description: not found
+//		'406':
+//			description: not acceptable
+//		'500':
+//			description: internal server error
+func (m *Module) ScheduledStatusesGETHandler(c *gin.Context) {
+	authed, errWithCode := apiutil.TokenAuth(c,
+		true, true, true, true,
+		apiutil.ScopeReadStatuses,
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		apiutil.ErrorHandler(c, gtserror.NewErrorNotAcceptable(err, err.Error()), m.processor.InstanceGetV1)
+		return
+	}
+
+	page, errWithCode := paging.ParseIDPage(c,
+		1,  // min limit
+		80, // max limit
+		20, // default limit
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	resp, errWithCode := m.processor.Status().ScheduledStatusesGetPage(
+		c.Request.Context(),
+		authed.Account,
+		page,
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if resp.LinkHeader != "" {
+		c.Header("Link", resp.LinkHeader)
+	}
+
+	apiutil.JSON(c, http.StatusOK, resp.Items)
+}