Phase 1: SQLite persistence layer

- Add modernc.org/sqlite (pure Go, no CGO)
- db/ package: trades, orders, config_log tables + CRUD
- Trade persistence: every closed trade saved to SQLite
- Restart recovery: open positions restored from DB
- Automatic migration on startup

db/db.go

@@ -0,0 +1,109 @@
+package db
+
+import (
+	"database/sql"
+	"log"
+	"os"
+	"path/filepath"
+	"time"
+
+	_ "modernc.org/sqlite"
+)
+
+// DB wraps the sql.DB connection with our schema.
+type DB struct {
+	*sql.DB
+	Path string
+}
+
+// Open opens or creates the SQLite database at the given path.
+func Open(path string) (*DB, error) {
+	if path == "" {
+		home, _ := os.UserHomeDir()
+		path = filepath.Join(home, "Project", "exchange-monitor-go", "data", "trades.db")
+	}
+
+	// Ensure directory exists
+	os.MkdirAll(filepath.Dir(path), 0755)
+
+	db, err := sql.Open("sqlite", path)
+	if err != nil {
+		return nil, err
+	}
+	db.SetMaxOpenConns(1) // SQLite single-writer
+	db.SetMaxIdleConns(1)
+
+	d := &DB{DB: db, Path: path}
+	if err := d.migrate(); err != nil {
+		return nil, err
+	}
+	return d, nil
+}
+
+func (d *DB) migrate() error {
+	schema := `
+	CREATE TABLE IF NOT EXISTS trades (
+		id           INTEGER PRIMARY KEY AUTOINCREMENT,
+		coin         TEXT NOT NULL,
+		direction    TEXT NOT NULL,
+		status       TEXT NOT NULL DEFAULT 'open',
+		entry_spread REAL,
+		exit_spread  REAL,
+		long_exchange TEXT,
+		short_exchange TEXT,
+		long_entry   REAL,
+		long_exit    REAL,
+		short_entry  REAL,
+		short_exit   REAL,
+		long_pnl     REAL,
+		short_pnl    REAL,
+		fee_entry    REAL,
+		fee_exit     REAL,
+		net_pnl      REAL,
+		amount_usd   REAL,
+		scale_count  INTEGER DEFAULT 0,
+		exit_reason  TEXT,
+		convergence  TEXT,
+		opened_at    DATETIME NOT NULL,
+		closed_at    DATETIME
+	);
+
+	CREATE TABLE IF NOT EXISTS orders (
+		id         INTEGER PRIMARY KEY AUTOINCREMENT,
+		trade_id   INTEGER NOT NULL REFERENCES trades(id),
+		leg        TEXT NOT NULL,
+		type       TEXT NOT NULL,
+		exchange   TEXT NOT NULL,
+		side       TEXT NOT NULL,
+		price      REAL,
+		size       REAL,
+		fee        REAL,
+		order_id   TEXT,
+		status     TEXT,
+		created_at DATETIME NOT NULL
+	);
+
+	CREATE TABLE IF NOT EXISTS config_log (
+		id         INTEGER PRIMARY KEY AUTOINCREMENT,
+		key        TEXT NOT NULL,
+		value      TEXT NOT NULL,
+		changed_at DATETIME NOT NULL
+	);
+
+	CREATE INDEX IF NOT EXISTS idx_trades_coin     ON trades(coin);
+	CREATE INDEX IF NOT EXISTS idx_trades_status   ON trades(status);
+	CREATE INDEX IF NOT EXISTS idx_trades_opened   ON trades(opened_at);
+	CREATE INDEX IF NOT EXISTS idx_orders_trade_id ON orders(trade_id);
+	`
+	_, err := d.Exec(schema)
+	if err != nil {
+		return err
+	}
+	log.Printf("[DB] SQLite ready: %s", d.Path)
+	return nil
+}
+
+// Now returns the current time in UTC.
+func Now() time.Time {
+	return time.Now().UTC()
+}