feat: implement real admin authentication and password management

- Added 'users' table to database with bcrypt hashing.
- Refactored login to verify against the database.
- Implemented 'Security' section in settings to allow changing the admin password.
- Initialized system with default user 'admin' and password 'admin'.

Cargo.toml

@@ -46,6 +46,7 @@ mime_guess = "2.0"
 # ========== Error Handling & Utilities ==========
 anyhow = "1.0"
 thiserror = "1.0"
+bcrypt = "0.15"
 chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1.0", features = ["v4", "serde"] }
 futures = "0.3"