fix: prevent UNIQUE constraint race with gunicorn multi-worker

Use INSERT OR IGNORE instead of SELECT-then-INSERT for
default admin user creation, avoiding the race condition
when multiple gunicorn workers initialize simultaneously.

README.md

@@ -80,6 +80,8 @@ uv add gunicorn
 uv run gunicorn -w 4 -b 0.0.0.0:5000 app:app
 ```
 
+> **Note:** The `-b` flag is required because gunicorn is a separate WSGI server and does not read Flask's `config.py`. The values in `config.py` (`HOST`, `PORT`) only apply to Flask's built-in dev server (`app.run()`).
+
 ## Project Structure
 
 ```

schema.py

@@ -86,16 +86,14 @@ def init_db():
         conn.commit()
 
         import bcrypt
-        cur = conn.execute("SELECT COUNT(*) FROM users WHERE role = 'admin'")
-        if cur.fetchone()[0] == 0:
-            password_hash = bcrypt.hashpw(
-                config.DEFAULT_ADMIN_PASSWORD.encode("utf-8"),
-                bcrypt.gensalt()
-            ).decode("utf-8")
-            conn.execute(
-                "INSERT INTO users (username, password_hash, role, max_goals) VALUES (?, ?, ?, ?)",
-                (config.DEFAULT_ADMIN_USERNAME, password_hash, "admin", 100)
-            )
-            conn.commit()
+        password_hash = bcrypt.hashpw(
+            config.DEFAULT_ADMIN_PASSWORD.encode("utf-8"),
+            bcrypt.gensalt()
+        ).decode("utf-8")
+        conn.execute(
+            "INSERT OR IGNORE INTO users (username, password_hash, role, max_goals) VALUES (?, ?, ?, ?)",
+            (config.DEFAULT_ADMIN_USERNAME, password_hash, "admin", 100)
+        )
+        conn.commit()
     finally:
         conn.close()