feat: add email provider configuration management

Merged Nguyễn Ngọc Đại requested to merge feature/email-config into main

Introduce a new feature for managing tenant-level email provider configurations (SMTP, IMAP, etc.) with full CRUD, activation, and test-connection capabilities.

Changes:

  • Add alembic migration creating email_provider_configs table with tenant_id FK, JSONB config column, is_active flag, and indexes
  • Add API router for email provider configs under /email-providers exposing list, create, get, update, delete, activate, and test-connection endpoints
  • Add Pydantic schemas (create/update/response, test-connection request/response) with field-level validation
  • Add SQLAlchemy model and service layer with tenant scoping and single-active-config enforcement per tenant
  • Register new router in v1 API aggregator

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