feat: implement SQLite persistent storage for analysis results

- Implemented database initialization and connection management in `database.ts` using Bun's SQLite.
- Created `runs` and `results` tables to track historical analysis metadata and detailed product performance.
- Updated `writer.ts` to persist analysis results to the database within a transaction, replacing the previous CSV output logic.
- Updated README and `.gitignore` to reflect the new persistent storage capability.

src/database.ts

@@ -0,0 +1,80 @@
+import { Database } from "bun:sqlite";
+
+let db: Database | null = null;
+
+export function getDb(dbPath: string): Database {
+  if (!db) {
+    db = new Database(dbPath);
+    db.run("PRAGMA journal_mode = WAL;"); // Enable WAL mode for better performance
+  }
+  return db;
+}
+
+export function closeDb(): void {
+  if (db) {
+    db.close();
+    db = null;
+  }
+}
+
+export function initDb(dbPath: string): void {
+  const database = getDb(dbPath);
+  database.run(`
+    CREATE TABLE IF NOT EXISTS runs (
+      id INTEGER PRIMARY KEY AUTOINCREMENT,
+      timestamp TEXT NOT NULL,
+      input_file TEXT NOT NULL,
+      output_file TEXT,
+      total_products INTEGER,
+      fba_count INTEGER,
+      fbm_count INTEGER,
+      skip_count INTEGER
+    );
+  `);
+  database.run(`
+    CREATE TABLE IF NOT EXISTS results (
+      id INTEGER PRIMARY KEY AUTOINCREMENT,
+      run_id INTEGER NOT NULL,
+      asin TEXT NOT NULL,
+      product_name TEXT,
+      brand TEXT,
+      category TEXT,
+      unit_cost REAL,
+      current_price REAL,
+      avg_price_90d REAL,
+      avg_price_90d_sheet REAL,
+      selling_price_sheet REAL,
+      sales_rank INTEGER,
+      rank_avg_90d INTEGER,
+      sellers INTEGER,
+      monthly_sold INTEGER,
+      rank_drops_30d INTEGER,
+      rank_drops_90d INTEGER,
+      fba_net_sheet REAL,
+      gross_profit_dollar REAL,
+      gross_profit_pct REAL,
+      net_profit_sheet REAL,
+      roi_sheet REAL,
+      moq INTEGER,
+      moq_cost REAL,
+      qty_available INTEGER,
+      supplier TEXT,
+      source_url TEXT,
+      asin_link TEXT,
+      promo_coupon_code TEXT,
+      notes TEXT,
+      lead_date TEXT,
+      fba_fee REAL,
+      fbm_fee REAL,
+      referral_percent REAL,
+      can_sell TEXT,
+      sellability_status TEXT,
+      sellability_reason TEXT,
+      verdict TEXT NOT NULL,
+      confidence INTEGER,
+      reasoning TEXT,
+      fetched_at TEXT NOT NULL,
+      FOREIGN KEY (run_id) REFERENCES runs(id)
+    );
+  `);
+}