feat: implement filament and spool preset services with CRUD operations

- Add `FilamentService` for managing filament inventory, including creation, retrieval, updating, and deletion of filaments.
- Introduce `SpoolPresetService` for handling spool presets, allowing users to create, update, and delete their custom presets.
- Create domain models for `Filament` and `SpoolPreset` with necessary fields and methods.
- Define repository interfaces for filament and spool preset persistence.
- Implement application configuration management from environment variables.
- Set up error handling with a centralized `AppError` type.
- Build the Axum router with public and protected routes for user authentication and resource management.

backend/Cargo.toml

@@ -0,0 +1,70 @@
+[package]
+name = "meowspool-backend"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "meowspool"
+path = "src/main.rs"
+
+[dependencies]
+# HTTP Framework
+axum = { version = "0.7", features = ["macros"] }
+tower = "0.4"
+tower-http = { version = "0.5", features = ["cors", "trace"] }
+
+# Async Runtime
+tokio = { version = "1", features = ["full"] }
+
+# Database
+sqlx = { version = "0.8", features = [
+    "runtime-tokio-rustls",
+    "postgres",
+    "uuid",
+    "time",
+    "macros",
+] }
+
+# Auth
+jsonwebtoken = "9"
+argon2 = "0.5"
+oauth2 = "4"
+
+# Serialization
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+# UUID
+uuid = { version = "1", features = ["v4", "serde"] }
+
+# Time
+time = { version = "0.3", features = ["serde", "serde-well-known"] }
+
+# Errors
+thiserror = "1"
+anyhow = "1"
+
+# Validation
+validator = { version = "0.18", features = ["derive"] }
+
+# Env vars
+dotenvy = "0.15"
+
+# Logging / Tracing
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+
+# HTTP Client (OAuth, Google APIs)
+reqwest = { version = "0.12", features = ["json", "rustls-tls"], default-features = false }
+
+# QR Code generation
+qrcode = "0.14"
+image = "0.25"
+base64 = "0.22"
+
+# Async trait
+async-trait = "0.1"
+
+[dev-dependencies]
+tokio = { version = "1", features = ["full"] }
+axum-test = "15"