Replace mock time-series client with real historical data client

[BytecodeBrewer]

Goal

Replace the mock time-series data client with a real historical data client selected from the research ticket.

Why

ARGUS needs real historical exchange-rate or market data to calculate meaningful analytics metrics. The mock client was useful for early prototyping, but Sprint 2 should move toward real data workflows.

The existing ExchangeRate API client should remain available for live conversion. The new client should provide historical time-series data for analytics.

Scope

• Add a new historical data client based on the selected source
• Fetch data for a configurable date range
• Return data in a pandas-friendly format
• Keep the client independent from GUI code
• Handle basic request and response errors
• Replace or reduce use of the mock client in analytics workflows
• Add tests with mocked HTTP/API responses

Acceptance criteria

• Real historical data can be fetched for a date range
• Output can be used by pandas-based analytics functions
• Existing ExchangeRate API live conversion remains available
• Mock client is no longer the main source for analytics data
• HTTP/API errors are handled
• Tests cover successful and failing responses
• No NiceGUI migration is included in this ticket

Note

Priority: Must