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>

db/schema.sql

@@ -0,0 +1,51 @@
+CREATE TABLE IF NOT EXISTS signals (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    coin TEXT NOT NULL,
+    timestamp DATETIME DEFAULT CURRENT_TIMESTAMP,
+    technical_score REAL DEFAULT 50,
+    news_score REAL DEFAULT 50,
+    social_score REAL DEFAULT 50,
+    ai_score REAL DEFAULT 50,
+    composite_score REAL DEFAULT 50,
+    signal TEXT DEFAULT 'HOLD'
+);
+
+CREATE TABLE IF NOT EXISTS trades (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    coin TEXT NOT NULL,
+    side TEXT NOT NULL,
+    price REAL NOT NULL,
+    quantity REAL NOT NULL,
+    amount_usd REAL NOT NULL,
+    timestamp DATETIME DEFAULT CURRENT_TIMESTAMP,
+    reason TEXT
+);
+
+CREATE TABLE IF NOT EXISTS positions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    coin TEXT NOT NULL,
+    entry_price REAL NOT NULL,
+    quantity REAL NOT NULL,
+    invested_usd REAL NOT NULL,
+    status TEXT DEFAULT 'OPEN',
+    opened_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    closed_at DATETIME
+);
+
+CREATE TABLE IF NOT EXISTS portfolio (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    timestamp DATETIME DEFAULT CURRENT_TIMESTAMP,
+    total_value REAL NOT NULL,
+    cash REAL NOT NULL,
+    pnl REAL NOT NULL,
+    pnl_pct REAL NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS settings (
+    key TEXT PRIMARY KEY,
+    value TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_signals_coin_ts ON signals(coin, timestamp);
+CREATE INDEX IF NOT EXISTS idx_positions_status ON positions(status);
+CREATE INDEX IF NOT EXISTS idx_trades_coin ON trades(coin);