Add unified posts database groundwork.

Introduce additive post and post-vote tables with an idempotent legacy snapshot migration so the existing application remains compatible during the staged cutover.

schema.sql

@@ -42,6 +42,38 @@ CREATE TABLE IF NOT EXISTS answers (
   updated_at TEXT NOT NULL
 );
 
+CREATE TABLE IF NOT EXISTS posts (
+  id TEXT PRIMARY KEY,
+  parent_id TEXT REFERENCES posts(id) ON DELETE CASCADE,
+  author_id TEXT NOT NULL REFERENCES users(id),
+  title TEXT NOT NULL DEFAULT '',
+  body TEXT NOT NULL,
+  city TEXT NOT NULL DEFAULT '',
+  hunt_date TEXT NOT NULL DEFAULT '',
+  hidden INTEGER NOT NULL DEFAULT 0,
+  created_at TEXT NOT NULL,
+  updated_at TEXT NOT NULL,
+  CHECK (
+    (parent_id IS NULL AND title <> '' AND hunt_date <> '')
+    OR
+    (parent_id IS NOT NULL AND title = '' AND city = '' AND hunt_date = '' AND hidden = 0)
+  )
+);
+
+CREATE INDEX IF NOT EXISTS idx_posts_parent_created
+  ON posts(parent_id, created_at, id);
+
+CREATE INDEX IF NOT EXISTS idx_posts_root_hunt
+  ON posts(hunt_date, hidden)
+  WHERE parent_id IS NULL;
+
+CREATE TABLE IF NOT EXISTS post_votes (
+  user_id TEXT NOT NULL REFERENCES users(id),
+  post_id TEXT NOT NULL REFERENCES posts(id) ON DELETE CASCADE,
+  value INTEGER NOT NULL CHECK (value IN (-1, 1)),
+  PRIMARY KEY (user_id, post_id)
+);
+
 CREATE TABLE IF NOT EXISTS sessions (
   token TEXT PRIMARY KEY,
   data BYTEA NOT NULL,