Adds Polaris Knowledge API as a third data vendor alongside yfinance and
Alpha Vantage. Polaris provides sentiment-scored intelligence, composite
trading signals, and news impact analysis that raw data feeds don't offer.
Implements all existing vendor methods plus 3 Polaris-exclusive methods
(sentiment_score, sector_analysis, news_impact).
Uses cachetools.TTLCache for thread-safe caching, shared financials fetch
to avoid redundant API calls, and polaris-news SDK for all API access.
Tested with real API calls — all functions return valid data.
Addresses #305, #11, #86.
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