Implemented v1.5 executable edge contract, added mode/maxDataAge controls, and harden caches/health/tests/docs

[ShinyPotatoo]

This Pull Request implements the v1.5 PRD/TDD signal API, including executable net edge calculation, stricter market matching, schema compliance, API contract, and caching behavior.
It also incorporates non-conflicting upstream changes from a related PR, while preserving the v1.5 signal model as the canonical behavior.
Request Path:
The /api/markets/arbitrage endpoint supports query parameters including mode, minNetEdgeBps / minSpread, maxDataAgeMs, and related filters.
Data Layer:
getMarkets() loads Polymarket and Kalshi markets using TTL caching with in-flight deduplication. Market freshness is derived and degrades when maxDataAgeMs is exceeded.
Computation:
detectArbitrage() groups markets by category, applies stricter matching rules, computes executable YES-side pricing, and derives both gross and net edge in bps, including fees, slippage, and latency adjustments. Liquidity and expiry filters are applied, and results are sorted deterministically.
Output Shape:
Returned opportunities conform to the v1.5 schema. In fast mode, a reduced payload is returned.
Observability:
Structured logs are emitted in the handler. Cache and freshness metadata are exposed where enabled.
Clients:
The SDK supports the new query parameters, and tests validate the contract against a live API instance.

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