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/migrations/20240101000003_create_filaments.sql

@@ -0,0 +1,26 @@
+-- Migration: 003 - Criar tabela de filamentos
+
+CREATE TABLE filaments (
+    id               UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    user_id          UUID NOT NULL REFERENCES users (id) ON DELETE CASCADE,
+    material         TEXT NOT NULL,
+    brand            TEXT NOT NULL,
+    model            TEXT,
+    color_hex        TEXT NOT NULL,
+    spool_preset_id  UUID NOT NULL REFERENCES spool_presets (id),
+    total_weight_g   INTEGER NOT NULL CHECK (total_weight_g > 0),
+    net_weight_g     INTEGER NOT NULL CHECK (net_weight_g >= 0),
+    temp_hotend_c    INTEGER CHECK (temp_hotend_c > 0),
+    temp_bed_c       INTEGER CHECK (temp_bed_c >= 0),
+    flow_factor_pct  DOUBLE PRECISION CHECK (flow_factor_pct > 0),
+    notes            TEXT,
+    updated_at       TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    created_at       TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_filaments_user_id   ON filaments (user_id);
+CREATE INDEX idx_filaments_material  ON filaments (user_id, material);
+CREATE INDEX idx_filaments_brand     ON filaments (user_id, brand);
+CREATE INDEX idx_filaments_updated_at ON filaments (user_id, updated_at DESC);
+-- Índice para suporte a alertas de estoque baixo
+CREATE INDEX idx_filaments_net_weight ON filaments (user_id, net_weight_g);