feat: add database layer with schema migrations

Add internal/database package with SQLite3 connection management,
versioned migration system, and three schema tables (artist_settings,
external_releases, notifications_sent). Includes 7 tests covering
initialization, migration idempotency, Close() behavior, and schema
validation using in-memory SQLite.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/database/database.go

@@ -0,0 +1,154 @@
+package database
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+// DB wraps sql.DB with migration support.
+type DB struct {
+	conn *sql.DB
+}
+
+// New opens a SQLite database at dbPath and runs schema migrations.
+func New(dbPath string) (*DB, error) {
+	conn, err := sql.Open("sqlite3", dbPath)
+	if err != nil {
+		return nil, fmt.Errorf("open database: %w", err)
+	}
+
+	// Enable WAL mode for better concurrent read performance.
+	if _, err := conn.Exec("PRAGMA journal_mode=WAL"); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("set WAL mode: %w", err)
+	}
+
+	db := &DB{conn: conn}
+	if err := db.migrate(); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("migrate: %w", err)
+	}
+
+	return db, nil
+}
+
+// Close closes the database connection.
+func (db *DB) Close() error {
+	return db.conn.Close()
+}
+
+// Conn returns the underlying sql.DB for use by other packages.
+func (db *DB) Conn() *sql.DB {
+	return db.conn
+}
+
+// migrate runs all pending schema migrations in order.
+func (db *DB) migrate() error {
+	// Create the migrations tracking table first, unconditionally.
+	if _, err := db.conn.Exec(`CREATE TABLE IF NOT EXISTS _migrations (
+		version INTEGER PRIMARY KEY,
+		name TEXT NOT NULL,
+		applied_at DATETIME DEFAULT CURRENT_TIMESTAMP
+	);`); err != nil {
+		return fmt.Errorf("create migrations table: %w", err)
+	}
+
+	migrations := []struct {
+		name string
+		sql  string
+	}{
+		{
+			name: "001_create_artist_settings",
+			sql: `CREATE TABLE IF NOT EXISTS artist_settings (
+				id TEXT PRIMARY KEY,
+				name TEXT NOT NULL,
+				ignore_singles BOOLEAN DEFAULT 0,
+				ignore_compilations BOOLEAN DEFAULT 0,
+				monitored BOOLEAN DEFAULT 1
+			);`,
+		},
+		{
+			name: "002_create_external_releases",
+			sql: `CREATE TABLE IF NOT EXISTS external_releases (
+				rgid TEXT PRIMARY KEY,
+				artist_id TEXT NOT NULL,
+				title TEXT NOT NULL,
+				type TEXT,
+				release_date TEXT,
+				is_ignored BOOLEAN DEFAULT 0
+			);`,
+		},
+		{
+			name: "003_create_notifications_sent",
+			sql: `CREATE TABLE IF NOT EXISTS notifications_sent (
+				rgid TEXT NOT NULL,
+				sent_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+				PRIMARY KEY (rgid, sent_at)
+			);`,
+		},
+	}
+
+	for _, m := range migrations {
+		applied, err := db.isMigrationApplied(m.name)
+		if err != nil {
+			return fmt.Errorf("check migration %s: %w", m.name, err)
+		}
+		if applied {
+			continue
+		}
+
+		if _, err := db.conn.Exec(m.sql); err != nil {
+			return fmt.Errorf("apply migration %s: %w", m.name, err)
+		}
+
+		if err := db.markMigrationApplied(m.name); err != nil {
+			return fmt.Errorf("record migration %s: %w", m.name, err)
+		}
+	}
+
+	return nil
+}
+
+// isMigrationApplied checks whether a migration with the given name has already been applied.
+func (db *DB) isMigrationApplied(name string) (bool, error) {
+	var count int
+	err := db.conn.QueryRow("SELECT COUNT(*) FROM _migrations WHERE name = ?", name).Scan(&count)
+	if err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+// markMigrationApplied records a migration as applied.
+func (db *DB) markMigrationApplied(name string) error {
+	_, err := db.conn.Exec("INSERT INTO _migrations (name) VALUES (?)", name)
+	return err
+}
+
+// ArtistSettings represents a row in the artist_settings table.
+type ArtistSettings struct {
+	ID                  string `json:"id"`
+	Name                string `json:"name"`
+	IgnoreSingles       bool   `json:"ignore_singles"`
+	IgnoreCompilations  bool   `json:"ignore_compilations"`
+	Monitored           bool   `json:"monitored"`
+}
+
+// ExternalRelease represents a row in the external_releases table.
+type ExternalRelease struct {
+	RGID         string    `json:"rgid"`
+	ArtistID     string    `json:"artist_id"`
+	Title        string    `json:"title"`
+	Type         string    `json:"type"`
+	ReleaseDate  string    `json:"release_date"`
+	IsIgnored    bool      `json:"is_ignored"`
+}
+
+// NotificationSent represents a row in the notifications_sent table.
+type NotificationSent struct {
+	RGID    string    `json:"rgid"`
+	SentAt  time.Time `json:"sent_at"`
+}