refactor: replace init_db with lightweight schema migration system

- Add MIGRATIONS list — each entry is a list of SQL statements for
  that schema version; append new lists to add future migrations,
  never modify existing ones
- Add schema_version table to track applied migrations
- migrate_db() runs on startup and applies any pending versions
  automatically; safe to run against existing DBs (v1 uses
  CREATE IF NOT EXISTS so it no-ops on the existing table/indexes)

app.py

@@ -51,12 +51,12 @@ def contains_banned_words(text):
             return True
     return False
 
-def init_db():
-    # TODO: No schema versioning — adding columns in the future requires manual DB updates. Consider a migration tool (e.g. Alembic).
-    conn = sqlite3.connect(DATABASE)
-    c = conn.cursor()
-    c.execute('''
-        CREATE TABLE IF NOT EXISTS guests (
+# Each entry is a list of SQL statements for that schema version.
+# To add a column or index in the future, append a new list — never modify existing entries.
+MIGRATIONS = [
+    # v1 — initial schema
+    [
+        '''CREATE TABLE IF NOT EXISTS guests (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             first_name TEXT NOT NULL,
             last_name TEXT NOT NULL,
@@ -65,13 +65,40 @@ def init_db():
             comment TEXT,
             newsletter_opt_in BOOLEAN DEFAULT 1,
             timestamp DATETIME DEFAULT CURRENT_TIMESTAMP
-        )
-    ''')
-    c.execute('CREATE INDEX IF NOT EXISTS idx_guests_id ON guests (id DESC)')
-    c.execute('CREATE INDEX IF NOT EXISTS idx_guests_email ON guests (email)')
-    conn.commit()
+        )''',
+        'CREATE INDEX IF NOT EXISTS idx_guests_id ON guests (id DESC)',
+        'CREATE INDEX IF NOT EXISTS idx_guests_email ON guests (email)',
+    ],
+]
+
+def migrate_db():
+    conn = sqlite3.connect(DATABASE)
+    c = conn.cursor()
+
+    # Bootstrap the version table and seed it at 0 if empty
+    c.execute('CREATE TABLE IF NOT EXISTS schema_version (version INTEGER NOT NULL)')
+    if c.execute('SELECT COUNT(*) FROM schema_version').fetchone()[0] == 0:
+        c.execute('INSERT INTO schema_version VALUES (0)')
+        conn.commit()
+
+    current = c.execute('SELECT version FROM schema_version').fetchone()[0]
+    pending = MIGRATIONS[current:]
+
+    if not pending:
+        logger.info("Database schema is up to date at v%d.", current)
+        conn.close()
+        return
+
+    for statements in pending:
+        current += 1
+        logger.info("Applying migration v%d...", current)
+        for sql in statements:
+            c.execute(sql)
+        c.execute('UPDATE schema_version SET version = ?', (current,))
+        conn.commit()
+
+    logger.info("Database migrated to v%d.", current)
     conn.close()
-    logger.info("Database initialized.")
 
 def is_valid_email(email):
     try:
@@ -81,7 +108,7 @@ def is_valid_email(email):
         return False
 
 with app.app_context():
-    init_db()
+    migrate_db()
 
 @app.route('/', methods=['GET', 'POST'])
 @limiter.limit("5 per minute", methods=["POST"])
@@ -186,6 +213,6 @@ def api_guests():
     return jsonify(guests)
 
 if __name__ == '__main__':
-    init_db()
+    migrate_db()
     logger.info("Starting development server at http://0.0.0.0:8000")
     app.run(host='0.0.0.0', port=8000)