Add local authentication option (#37)

Co-authored-by: Shaun Campbell <shaun@campbellwireless.net>
Reviewed-on: #37
Co-authored-by: AI Agent <ai-agent@campbellwireless.net>
Co-committed-by: AI Agent <ai-agent@campbellwireless.net>

README.md

@@ -43,6 +43,19 @@ bun install
 bun run dev
 ```
 
+## Local authentication
+
+Local auth is optional and off by default. Enable it with the env vars in `.env.example` and ensure users have a matching row in both `users` and `local_credentials`.
+
+To seed a local password hash, use Argon2id with the configured parameters and insert it into `local_credentials`:
+
+```sql
+INSERT INTO local_credentials (user_id, password_hash)
+VALUES ('<user-id>', '<argon2id hash>');
+```
+
+Passwords must be at least 12 characters. Avoid storing plaintext passwords anywhere.
+
 ## Quality and Tests
 
 ```sh