refactor: optimize project structure, migrate to SQLite, and add new features

fix:
  - Fix TypeScript type errors in api-wrapper.ts (ApiResponse type)
  - Fix backward compatibility in database.ts getSettings() for missing fields
  - Fix default value for weekNavigationEnabled (changed from true to false)
  - Fix API routes error handling with unified wrapper
  - Fix duplicate toggle switch code in admin.tsx (6 instances)
  - Fix inconsistent authentication check in API routes (unified with withAuth)
  - Fix error message text in loading-context.tsx (improved user experience)

add:
  - Add database.ts: SQLite database layer with better-sqlite3 for persistent storage
    * Groups management (CRUD operations)
    * Settings management with caching
    * Admin password hashing with bcrypt
    * Automatic database initialization and migration
  - Add api-wrapper.ts utility for unified API route handling
    * withAuth wrapper for protected routes
    * withMethods wrapper for public routes
    * Consistent error handling and method validation
  - Add validation.ts utility with centralized validation functions
    * validateCourse - course validation (1-5)
    * validateGroupId - group ID format validation
    * validatePassword - password strength validation
  - Add showAddGroupButton setting to control visibility of 'Add Group' button on homepage
  - Add toggle switch component in admin.tsx for reusable UI (replaces 6 duplicate instances)
  - Add CourseSelect component in admin.tsx for reusable course selection
  - Add DialogFooterButtons component in admin.tsx for reusable dialog footer
  - Add unified loadData function in admin.tsx to reduce code duplication
  - Add change-password.ts API endpoint for admin password management
  - Add logs.ts API endpoint for viewing error logs in admin panel
  - Add logErrorToFile function in logger.ts for persistent error logging
  - Add comprehensive error logging in schedule.ts (parsing, fetch, timeout, network errors)
  - Add comprehensive project structure documentation in README.md
  - Add architecture and code organization section in README.md
  - Add database information section in README.md
  - Add SQLite and bcrypt to tech stack documentation
  - Add better-sqlite3 and bcrypt dependencies to package.json
  - Add .gitignore rules for error.log and database files (data/, *.db, *.db-shm, *.db-wal)

refactor:
  - Refactor admin.tsx: extract reusable components (toggle, select, dialog footer)
  - Refactor API routes to use withAuth wrapper for consistent authentication
  - Refactor API routes to use validation utilities instead of inline validation
  - Refactor groups.ts and groups.json: move to old/data/ directory (deprecated, now using SQLite)
  - Refactor settings-loader.ts: migrate from JSON to SQLite database
  - Refactor groups-loader.ts: migrate from JSON to SQLite database
  - Refactor database.ts: improve backward compatibility for settings migration
  - Refactor admin.tsx: unify data loading functions (loadGroupsList, loadSettingsList)
  - Refactor index.tsx: add showAddGroupButton prop and conditional rendering
  - Refactor API routes: consistent error handling and method validation
  - Refactor README.md: update tech stack, project structure, and admin panel documentation
  - Refactor auth.ts: improve session management and cookie handling
  - Refactor schedule.ts: improve error handling with detailed logging and error types
  - Refactor logger.ts: add file-based error logging functionality
  - Refactor loading-context.tsx: improve error message clarity

remove:
  - Remove hello.ts test API endpoint
  - Remove groups.ts and groups.json (moved to old/data/, replaced by SQLite)

update:
  - Update .gitignore to exclude old data files, database files, and error logs
  - Update package.json: add better-sqlite3, bcrypt and their type definitions
  - Update README.md with new features, architecture, and database information
  - Update all API routes to use new wrapper system
  - Update admin panel with new settings and improved UI
  - Update sitemap.xml with cache usage comment

src/app/logger.ts

@@ -1,4 +1,6 @@
 import TelegramBot from 'node-telegram-bot-api'
+import fs from 'fs'
+import path from 'path'
 
 const token = process.env.PARSING_FAILURE_NOTIFICATIONS_TELEGRAM_BOTAPI_TOKEN
 const ownerID = process.env.PARSING_FAILURE_NOTIFICATIONS_TELEGRAM_CHAT_ID
@@ -10,6 +12,46 @@ if (!token || !ownerID) {
   bot = new TelegramBot(token, { polling: false })
 }
 
+// Путь к файлу логов (в корне проекта)
+const getErrorLogPath = () => {
+  // В production (standalone) используем текущую рабочую директорию
+  // В development используем корень проекта (process.cwd())
+  return path.join(process.cwd(), 'error.log')
+}
+
+/**
+ * Логирует ошибку в файл error.log
+ * @param error - Объект ошибки или строка с описанием ошибки
+ * @param context - Дополнительный контекст (опционально)
+ */
+export function logErrorToFile(error: Error | string, context?: Record<string, unknown>): void {
+  try {
+    const logPath = getErrorLogPath()
+    const timestamp = new Date().toISOString()
+    const errorMessage = error instanceof Error ? error.message : error
+    const errorStack = error instanceof Error ? error.stack : undefined
+    const errorName = error instanceof Error ? error.name : 'Error'
+    
+    let logEntry = `[${timestamp}] ${errorName}: ${errorMessage}\n`
+    
+    if (errorStack) {
+      logEntry += `Stack: ${errorStack}\n`
+    }
+    
+    if (context && Object.keys(context).length > 0) {
+      logEntry += `Context: ${JSON.stringify(context, null, 2)}\n`
+    }
+    
+    logEntry += '---\n'
+    
+    // Используем appendFileSync для надежности (не блокирует надолго)
+    fs.appendFileSync(logPath, logEntry, 'utf8')
+  } catch (logError) {
+    // Если не удалось записать в файл, выводим в консоль
+    console.error('Failed to write to error.log:', logError)
+  }
+}
+
 export async function reportParserError(...text: string[]) {
   if (!token || !ownerID) return