Refactor: Standardize Constants and Enforce Zero-Hardcode Architecture

Summary of Changes

This PR eliminates all hardcoded string literals and Prisma enum leakage across the backend codebase (data-crawler-be), adhering strictly to the layer separation and Prisma exclusivity rules defined in AGENTS.md.

Key Highlights

1. Centralized Constants Layer (src/common/constants/)

• Added CRAWL_MODE & CrawlMode (src/common/constants/crawl-mode.constant.ts): Standardized 'SCRAPE', 'CRAWL', 'SITEMAP', and 'URL_LIST'.
• Added SCHEDULE_FREQUENCY & ScheduleFrequency (src/common/constants/schedule-frequency.constant.ts): Standardized 'DAILY', 'WEEKLY', 'MONTHLY', and 'CUSTOM'.
• Added ASSET_TYPE & AssetType (src/common/constants/asset-type.constant.ts): Standardized 'IMAGE', 'LINK', 'PDF', 'FILE', 'VIDEO', and 'OTHER'.
• Added DEFAULT_TIMEZONE (src/common/constants/timezone.constant.ts): Centralized default timezone 'Asia/Ho_Chi_Minh'.
• Barrel Export (src/common/constants/index.ts): Re-exports all domain constants for single-point import.

2. Zero-Hardcode Refactoring

• Crawl Jobs & Worker (crawl.worker.processor.ts, crawl-job.service.ts, crawl-job.validation.ts):
  • Replaced hardcoded status strings with JOB_STATUS.* (COMPLETED, RUNNING, FAILED, CANCELED, etc.).
  • Replaced mode comparisons and array definitions with CRAWL_MODE.* and z.nativeEnum(CRAWL_MODE).
  • Replaced export type strings with EXPORT_TYPE.* and z.nativeEnum(EXPORT_TYPE).
• Crawl Schedules (crawl-schedule.service.ts, crawl-schedule.validation.ts, crawl-schedule.repository.ts):
  • Replaced hardcoded timezone 'Asia/Ho_Chi_Minh' with DEFAULT_TIMEZONE.
  • Replaced hardcoded schedule frequencies with SCHEDULE_FREQUENCY.* and z.nativeEnum(SCHEDULE_FREQUENCY).
• Users & Auth (auth.service.ts, user.service.ts, user.validation.ts):
  • Replaced 'ADMIN' literal with ROLES.ADMIN.
  • Used z.nativeEnum(ROLES) in user query and creation schemas.
  • Replaced hardcoded timezone with DEFAULT_TIMEZONE.
• Exports & Dashboard (export.service.ts, crawl-export.service.ts, dashboard.repository.ts):
  • Bound export handler keys using [EXPORT_TYPE.*].
  • Replaced hardcoded status counts with JOB_STATUS.*.

3. Strict Prisma Exclusivity Enforcement (AGENTS.md §1)

• Removed @prisma/client enum imports (UserRole, CrawlMode, CrawlJobStatus, ScheduleFrequency, ExportType, AssetType) from non-repository layers (Services, Controllers, DTOs, and Middlewares).
• Isolated DTO and Service typing strictly through domain constant types.

4. Documentation & Guidelines (AGENTS.md)

• Added the Zero Hardcode Principle under Section 1 invariant rules, providing clear declarations, validation patterns, and usage code examples for future development.