[feature] Implement following hashtags (#3141)

* Implement followed tags API

* Insert statuses with followed tags into home timelines

* Test following and unfollowing tags

* Correct Swagger path params

* Trim conversation caches

* Migration for followed_tags table

* Followed tag caches and DB implementation

* Lint and tests

* Add missing tag info endpoint, reorganize tag API

* Unwrap boosts when timelining based on tags

* Apply visibility filters to tag followers

* Address review comments

internal/cache/db.go

@@ -29,6 +29,9 @@ type DBCaches struct {
 	// Account provides access to the gtsmodel Account database cache.
 	Account StructCache[*gtsmodel.Account]
 
+	// AccountIDsFollowingTag caches account IDs following a given tag ID.
+	AccountIDsFollowingTag SliceCache[string]
+
 	// AccountNote provides access to the gtsmodel Note database cache.
 	AccountNote StructCache[*gtsmodel.AccountNote]
 
@@ -160,6 +163,9 @@ type DBCaches struct {
 	// Tag provides access to the gtsmodel Tag database cache.
 	Tag StructCache[*gtsmodel.Tag]
 
+	// TagIDsFollowedByAccount caches tag IDs followed by a given account ID.
+	TagIDsFollowedByAccount SliceCache[string]
+
 	// ThreadMute provides access to the gtsmodel ThreadMute database cache.
 	ThreadMute StructCache[*gtsmodel.ThreadMute]
 
@@ -234,6 +240,17 @@ func (c *Caches) initAccount() {
 	})
 }
 
+func (c *Caches) initAccountIDsFollowingTag() {
+	// Calculate maximum cache size.
+	cap := calculateSliceCacheMax(
+		config.GetCacheAccountIDsFollowingTagMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	c.DB.AccountIDsFollowingTag.Init(0, cap)
+}
+
 func (c *Caches) initAccountNote() {
 	// Calculate maximum cache size.
 	cap := calculateResultCacheMax(
@@ -1317,6 +1334,17 @@ func (c *Caches) initTag() {
 	})
 }
 
+func (c *Caches) initTagIDsFollowedByAccount() {
+	// Calculate maximum cache size.
+	cap := calculateSliceCacheMax(
+		config.GetCacheTagIDsFollowedByAccountMemRatio(),
+	)
+
+	log.Infof(nil, "cache size = %d", cap)
+
+	c.DB.TagIDsFollowedByAccount.Init(0, cap)
+}
+
 func (c *Caches) initThreadMute() {
 	cap := calculateResultCacheMax(
 		sizeofThreadMute(), // model in-mem size.