Replace scs postgresstore with a sqlc-backed SessionStore.

Keep scs for cookies and session API while sessions DDL and queries live in the same sqlc stack as the rest of Postgres.

internal/store/sqlc/models.go

@@ -4,6 +4,10 @@
 
 package sqlc
 
+import (
+	"time"
+)
+
 type Answer struct {
 	QuestionID string
 	AuthorID   string
@@ -23,6 +27,12 @@ type Question struct {
 	CreatedAt string
 }
 
+type Session struct {
+	Token  string
+	Data   []byte
+	Expiry time.Time
+}
+
 type User struct {
 	ID           string
 	Username     string