feat: initial implementation of metadata aggregator

- gRPC service with MusicBrainz provider
- PostgreSQL schema with migrations
- Service layer with database-first caching
- Repository pattern for data access
- YAML configuration support
- Research documentation for 17 music metadata projects

internal/config/config.go

@@ -0,0 +1,68 @@
+package config
+
+import (
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+type Config struct {
+	Server   ServerConfig   `yaml:"server"`
+	Database DatabaseConfig `yaml:"database"`
+}
+
+type ServerConfig struct {
+	Port int `yaml:"port"`
+}
+
+type DatabaseConfig struct {
+	Host     string `yaml:"host"`
+	Port     int    `yaml:"port"`
+	User     string `yaml:"user"`
+	Password string `yaml:"password"`
+	Name     string `yaml:"name"`
+	SSLMode  string `yaml:"sslmode"`
+}
+
+func Load(path string) (*Config, error) {
+	cfg := &Config{
+		Server: ServerConfig{
+			Port: 50051,
+		},
+		Database: DatabaseConfig{
+			Host:    "localhost",
+			Port:    5432,
+			SSLMode: "disable",
+		},
+	}
+
+	if path == "" {
+		return cfg, nil
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return cfg, nil
+		}
+		return nil, fmt.Errorf("read config: %w", err)
+	}
+
+	if err := yaml.Unmarshal(data, cfg); err != nil {
+		return nil, fmt.Errorf("parse config: %w", err)
+	}
+
+	return cfg, nil
+}
+
+func (d *DatabaseConfig) DSN() string {
+	if d.Host == "" || d.User == "" || d.Name == "" {
+		return ""
+	}
+
+	return fmt.Sprintf(
+		"postgres://%s:%s@%s:%d/%s?sslmode=%s",
+		d.User, d.Password, d.Host, d.Port, d.Name, d.SSLMode,
+	)
+}