Implement per board post number incrementing

internal/database/seed.sql

@@ -23,6 +23,7 @@ CREATE TABLE IF NOT EXISTS threads (
 CREATE TABLE IF NOT EXISTS posts (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     thread_id INTEGER NOT NULL,
+    number INTEGER NOT NULL,
     author TEXT DEFAULT 'Anonymous',
     body TEXT NOT NULL,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,