refactor(arch): split database.js into migrations + connection module (#57)

- database.js: 292→42 lines (connection + async helpers only)
- 001_initial_schema.js: 7 CREATE TABLE statements in transaction
- 002_add_columns.js: 5 ALTER TABLE checks with checkColumnExists
- 003_add_indexes.js: 6 CREATE INDEX statements
- runner.js: versioned migration runner with _meta table
- index.js: calls runMigrations() + cleanUpInvalidForeignKeys()
- ALLOWED_TABLES whitelist preserved in runner.js
- Schema version tracked in _meta table for idempotent runs

src/migrations/runner.js

@@ -0,0 +1,56 @@
+import db from '../config/database.js';
+
+const ALLOWED_TABLES = new Set([
+  'users', 'crypto_wallets', 'transactions', 'products',
+  'purchases', 'locations', 'categories'
+]);
+
+export const checkColumnExists = async (tableName, columnName) => {
+  if (!ALLOWED_TABLES.has(tableName)) {
+    throw new Error(`Invalid table name: ${tableName}`);
+  }
+  try {
+    const result = await db.allAsync(`PRAGMA table_info(${tableName})`);
+    return result.some(column => column.name === columnName);
+  } catch (error) {
+    console.error(`Error checking column ${columnName} in table ${tableName}:`, error);
+    return false;
+  }
+};
+
+export const cleanUpInvalidForeignKeys = async () => {
+  try {
+    await db.runAsync(`DELETE FROM crypto_wallets WHERE user_id NOT IN (SELECT id FROM users)`);
+    console.log('Cleaned up invalid foreign key references in crypto_wallets table');
+  } catch (error) {
+    console.error('Error cleaning up invalid foreign key references:', error);
+  }
+};
+
+export async function runMigrations() {
+  await db.runAsync(`CREATE TABLE IF NOT EXISTS _meta (key TEXT PRIMARY KEY, value TEXT)`);
+
+  const row = await db.getAsync(`SELECT value FROM _meta WHERE key = 'schema_version'`);
+  const currentVersion = row ? parseInt(row.value, 10) : 0;
+
+  const migrations = [
+    (await import('./001_initial_schema.js')).default,
+    (await import('./002_add_columns.js')).default,
+    (await import('./003_add_indexes.js')).default,
+  ];
+
+  for (let i = currentVersion; i < migrations.length; i++) {
+    console.log(`Running migration ${i + 1}/${migrations.length}...`);
+    if (i === 1) {
+      await migrations[i](db, checkColumnExists);
+    } else {
+      await migrations[i](db);
+    }
+    await db.runAsync(
+      `INSERT OR REPLACE INTO _meta (key, value) VALUES ('schema_version', ?)`,
+      [String(i + 1)]
+    );
+  }
+
+  console.log(`Migrations complete. Schema version: ${migrations.length}`);
+}