Build the NestJS economic calendar integration #91

Parent: #89

Goal

Fetch and normalize external economic-calendar events through the Tradel API.

Tasks

• Add a NestJS economic-calendar module, controller, service, and provider client.
• Validate the provider API key and related settings through the existing config flow.
• Add a documented endpoint for economic events and the supported filters.
• Keep the provider key on the server.
• Add request timeouts, safe retry behavior, cache handling, deduplication, and clear error mapping.
• Return event timestamps consistently in UTC.
• Validate query input and sanitize all data returned to the web app.
• Add unit tests for event mapping and failure cases, plus endpoint tests for the main flow.
• Update Swagger and local setup documentation.

Acceptance criteria

• The endpoint returns the agreed normalized event contract.
• Date, country, currency, impact, and other agreed filters work as documented.
• Provider failures and rate limits return safe, useful responses.
• Repeated requests use the agreed cache behavior.
• Event times are returned in UTC.
• Secrets never appear in responses or logs.
• Tests cover success, invalid input, timeout, provider error, and rate-limit cases.