Add database layer with SQLite schema, ORM-style Database class, and tests

Implements Task 2: creates db/schema.sql with five tables (signals, trades,
positions, portfolio, settings) and indexes; data/db.py with a Database class
covering all CRUD operations; tests/test_db.py with 6 passing pytest tests.

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

tests/test_db.py

@@ -0,0 +1,51 @@
+import os
+import pytest
+from data.db import Database
+
+@pytest.fixture
+def db(tmp_path):
+    db_path = str(tmp_path / "test.db")
+    database = Database(db_path)
+    database.init()
+    yield database
+    database.close()
+
+def test_init_creates_tables(db):
+    tables = db.execute("SELECT name FROM sqlite_master WHERE type='table'").fetchall()
+    names = {r[0] for r in tables}
+    assert "signals" in names
+    assert "trades" in names
+    assert "positions" in names
+    assert "portfolio" in names
+    assert "settings" in names
+
+def test_insert_signal(db):
+    db.insert_signal("BTCUSDT", 75.0, 60.0, 55.0, 70.0, 68.5, "HOLD")
+    rows = db.get_latest_signals()
+    assert len(rows) == 1
+    assert rows[0]["coin"] == "BTCUSDT"
+    assert rows[0]["composite_score"] == 68.5
+
+def test_insert_trade(db):
+    db.insert_trade("ETHUSDT", "BUY", 3500.0, 0.01, 35.0, "signal")
+    trades = db.get_trades()
+    assert len(trades) == 1
+    assert trades[0]["coin"] == "ETHUSDT"
+
+def test_open_close_position(db):
+    db.open_position("SOLUSDT", 140.0, 0.5, 70.0)
+    positions = db.get_open_positions()
+    assert len(positions) == 1
+    db.close_position(positions[0]["id"])
+    assert len(db.get_open_positions()) == 0
+
+def test_save_load_setting(db):
+    db.save_setting("weights", '{"technical": 0.7}')
+    val = db.load_setting("weights")
+    assert val == '{"technical": 0.7}'
+
+def test_portfolio_snapshot(db):
+    db.insert_portfolio_snapshot(210.0, 50.0, 10.0, 5.0)
+    snaps = db.get_portfolio_history()
+    assert len(snaps) == 1
+    assert snaps[0]["total_value"] == 210.0