Replace hidden flag with post state.

Store post state as text so Go owns the allowed values and future states such as locked remain representable without a database enum migration.

schema.sql

@@ -50,13 +50,13 @@ CREATE TABLE IF NOT EXISTS posts (
   body TEXT NOT NULL,
   city TEXT NOT NULL DEFAULT '',
   post_date TEXT NOT NULL DEFAULT '',
-  hidden INTEGER NOT NULL DEFAULT 0,
+  post_state TEXT NOT NULL,
   created_at TEXT NOT NULL,
   updated_at TEXT NOT NULL,
-  CHECK (
+  CONSTRAINT posts_shape_check CHECK (
     (parent_id IS NULL AND title <> '' AND post_date <> '')
     OR
-    (parent_id IS NOT NULL AND title = '' AND city = '' AND post_date = '' AND hidden = 0)
+    (parent_id IS NOT NULL AND title = '' AND city = '' AND post_date = '')
   )
 );
 
@@ -64,7 +64,7 @@ CREATE INDEX IF NOT EXISTS idx_posts_parent_created
   ON posts(parent_id, created_at, id);
 
 CREATE INDEX IF NOT EXISTS idx_posts_root_date
-  ON posts(post_date, hidden)
+  ON posts(post_date, post_state)
   WHERE parent_id IS NULL;
 
 CREATE TABLE IF NOT EXISTS post_votes (