Add SQLite visitor IP logging to /server_storage

Log visitor IPs and paths to a local SQLite database via the existing
analytics API route, persisted at /server_storage/visitors.db in Docker
(falls back to ./visitors.db in dev).

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

lib/db.ts

@@ -0,0 +1,28 @@
+import Database from 'better-sqlite3';
+import path from 'path';
+import fs from 'fs';
+
+const SERVER_STORAGE = '/server_storage';
+const dbPath = fs.existsSync(SERVER_STORAGE)
+  ? path.join(SERVER_STORAGE, 'visitors.db')
+  : path.join(process.cwd(), 'visitors.db');
+
+const db = new Database(dbPath);
+db.pragma('journal_mode = WAL');
+
+db.exec(`
+  CREATE TABLE IF NOT EXISTS visits (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    ip_address TEXT NOT NULL,
+    path TEXT NOT NULL,
+    visited_at TEXT NOT NULL DEFAULT (datetime('now'))
+  );
+`);
+
+const insertStmt = db.prepare(
+  'INSERT INTO visits (ip_address, path) VALUES (?, ?)'
+);
+
+export function logVisit(ip: string, visitPath: string) {
+  insertStmt.run(ip, visitPath);
+}