[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/cache/db.go

@@ -219,6 +219,9 @@ type DBCaches struct {
 	// Report provides access to the gtsmodel Report database cache.
 	Report StructCache[*gtsmodel.Report]
 
+	// ScheduledStatus provides access to the gtsmodel ScheduledStatus database cache.
+	ScheduledStatus StructCache[*gtsmodel.ScheduledStatus]
+
 	// SinBinStatus provides access to the gtsmodel SinBinStatus database cache.
 	SinBinStatus StructCache[*gtsmodel.SinBinStatus]
 
@@ -1287,6 +1290,40 @@ func (c *Caches) initReport() {
 	})
 }
 
+func (c *Caches) initScheduledStatus() {
+	// Calculate maximum cache size.
+	cap := calculateResultCacheMax(
+		sizeofScheduledStatus(), // model in-mem size.
+		config.GetCacheScheduledStatusMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	copyF := func(s1 *gtsmodel.ScheduledStatus) *gtsmodel.ScheduledStatus {
+		s2 := new(gtsmodel.ScheduledStatus)
+		*s2 = *s1
+
+		// Don't include ptr fields that
+		// will be populated separately.
+		s2.Account = nil
+		s2.Application = nil
+		s2.MediaAttachments = nil
+
+		return s2
+	}
+
+	c.DB.ScheduledStatus.Init(structr.CacheConfig[*gtsmodel.ScheduledStatus]{
+		Indices: []structr.IndexConfig{
+			{Fields: "ID"},
+			{Fields: "AccountID", Multiple: true},
+		},
+		MaxSize:    cap,
+		IgnoreErr:  ignoreErrors,
+		Copy:       copyF,
+		Invalidate: c.OnInvalidateScheduledStatus,
+	})
+}
+
 func (c *Caches) initSinBinStatus() {
 	// Calculate maximum cache size.
 	cap := calculateResultCacheMax(