docs(ingestor): create technical documentation for Ingestor POC (Issue #12)

[KaruG1999]

Description

This PR delivers the comprehensive README.md for the Ingestor Service, fulfilling the requirements of Issue #12.

It provides the architectural overview, configuration guide, and API specifications necessary for new contributors to onboard quickly.

Context & Dependencies

This documentation is written to reflect the complete functional state of the POC, combining:

1. Current Infrastructure: The SchedulerService (recently merged to main) for automatic fetching.
2. Pending Feature: The FinnhubAdapter (from PR Feat/stock provider adapter 8 #15), anticipating its merge to ensure the docs describe the intended data provider rather than the development mock.

Key Documentation Features

• Architecture: Visualizes the hybrid flow: Scheduled (Cron) -> PriceFetcher and On-Demand (HTTP) -> Controller.
• Configuration:
  • Explicitly documents the FINNHUB_API_KEY requirement (aligned with PR Feat/stock provider adapter 8 #15).
  • Adds new Scheduler variables: FETCH_INTERVAL_MS and STOCK_SYMBOLS.
• API Spec: Provides JSON examples for the normalized data structure.
• POC Limitations: Clearly lists constraints (No DB, No Retries, Single Provider) to manage expectations.

Verification

• Verified npm run lint compliance.
• Excluded Testing Instructions: Unit/Integration tests are scoped for Issue Integration Tests and E2E Testing #23 and are not yet implemented. The docs strictly reflect current capabilities.
• Verified consistency between the Scheduler logic in main and the Finnhub requirements.

Closes #12

[KaruG1999]

pd: cualquier cosita que agreguen que no haya tenido en cuenta me avisan, no tengo drama de modificarlo :) PV

[Villarley]

LGTM