feat: add DeFi data via DefiLlama API (TDD)

7 new endpoints under /api/v1/defi/ (all free, no API key):
- GET /defi/tvl/protocols - top DeFi protocols by TVL
- GET /defi/tvl/chains - chain TVL rankings
- GET /defi/tvl/{protocol} - single protocol TVL
- GET /defi/yields - top yield pools (filter by chain/project)
- GET /defi/stablecoins - stablecoin market data
- GET /defi/volumes/dexs - DEX volume overview
- GET /defi/fees - protocol fees/revenue overview

Data source: DefiLlama API (free, no key needed)
58 new tests (33 service + 25 route). All 561 tests passing.

main.py

@@ -40,6 +40,7 @@ from routes_technical import router as technical_router  # noqa: E402
 from routes_portfolio import router as portfolio_router  # noqa: E402
 from routes_backtest import router as backtest_router  # noqa: E402
 from routes_cn import router as cn_router  # noqa: E402
+from routes_defi import router as defi_router  # noqa: E402
 
 logging.basicConfig(
     level=settings.log_level.upper(),
@@ -87,6 +88,7 @@ app.include_router(regulators_router)
 app.include_router(portfolio_router)
 app.include_router(backtest_router)
 app.include_router(cn_router)
+app.include_router(defi_router)
 
 
 @app.get("/health", response_model=dict[str, str])