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>

data/db.py

@@ -0,0 +1,99 @@
+import sqlite3
+import os
+
+class Database:
+    def __init__(self, db_path: str):
+        self.db_path = db_path
+        self.conn = None
+
+    def init(self):
+        self.conn = sqlite3.connect(self.db_path, check_same_thread=False)
+        self.conn.execute("PRAGMA journal_mode=WAL")
+        self.conn.execute("PRAGMA busy_timeout=5000")
+        self.conn.row_factory = sqlite3.Row
+        schema_path = os.path.join(os.path.dirname(os.path.dirname(__file__)), "db", "schema.sql")
+        with open(schema_path) as f:
+            self.conn.executescript(f.read())
+
+    def close(self):
+        if self.conn:
+            self.conn.close()
+
+    def execute(self, sql, params=()):
+        return self.conn.execute(sql, params)
+
+    def insert_signal(self, coin, technical, news, social, ai, composite, signal):
+        self.conn.execute(
+            "INSERT INTO signals (coin, technical_score, news_score, social_score, ai_score, composite_score, signal) VALUES (?,?,?,?,?,?,?)",
+            (coin, technical, news, social, ai, composite, signal),
+        )
+        self.conn.commit()
+
+    def get_latest_signals(self, limit=50):
+        return self.conn.execute(
+            "SELECT * FROM signals WHERE id IN (SELECT MAX(id) FROM signals GROUP BY coin) ORDER BY composite_score DESC LIMIT ?",
+            (limit,),
+        ).fetchall()
+
+    def insert_trade(self, coin, side, price, quantity, amount_usd, reason):
+        self.conn.execute(
+            "INSERT INTO trades (coin, side, price, quantity, amount_usd, reason) VALUES (?,?,?,?,?,?)",
+            (coin, side, price, quantity, amount_usd, reason),
+        )
+        self.conn.commit()
+
+    def get_trades(self, limit=100):
+        return self.conn.execute(
+            "SELECT * FROM trades ORDER BY timestamp DESC LIMIT ?", (limit,)
+        ).fetchall()
+
+    def open_position(self, coin, entry_price, quantity, invested_usd):
+        self.conn.execute(
+            "INSERT INTO positions (coin, entry_price, quantity, invested_usd) VALUES (?,?,?,?)",
+            (coin, entry_price, quantity, invested_usd),
+        )
+        self.conn.commit()
+
+    def get_open_positions(self):
+        return self.conn.execute(
+            "SELECT * FROM positions WHERE status='OPEN'"
+        ).fetchall()
+
+    def close_position(self, position_id):
+        self.conn.execute(
+            "UPDATE positions SET status='CLOSED', closed_at=CURRENT_TIMESTAMP WHERE id=?",
+            (position_id,),
+        )
+        self.conn.commit()
+
+    def update_position_quantity(self, position_id, new_quantity):
+        self.conn.execute(
+            "UPDATE positions SET quantity=? WHERE id=?",
+            (new_quantity, position_id),
+        )
+        self.conn.commit()
+
+    def insert_portfolio_snapshot(self, total_value, cash, pnl, pnl_pct):
+        self.conn.execute(
+            "INSERT INTO portfolio (total_value, cash, pnl, pnl_pct) VALUES (?,?,?,?)",
+            (total_value, cash, pnl, pnl_pct),
+        )
+        self.conn.commit()
+
+    def get_portfolio_history(self, limit=100):
+        return self.conn.execute(
+            "SELECT * FROM portfolio ORDER BY timestamp DESC LIMIT ?", (limit,)
+        ).fetchall()
+
+    def save_setting(self, key, value):
+        self.conn.execute(
+            "INSERT OR REPLACE INTO settings (key, value) VALUES (?,?)",
+            (key, value),
+        )
+        self.conn.commit()
+
+    def load_setting(self, key):
+        row = self.conn.execute(
+            "SELECT value FROM settings WHERE key=?", (key,)
+        ).fetchone()
+        return row["value"] if row else None