feat(signal_v2): stock_client + 6 integration tests

httpx async client with custom retry loop (max 3, exponential 1s/2s/4s),
memory dict cache (portfolio 60s / news-sentiment 300s / screener 60s),
X-WebAI-Key auth header injection. Stale fallback returns last
successful response with logger.warning on persistent failures.

6 integration tests pass with respx httpx mock.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -47,9 +47,11 @@ daily_trade_history.json
 watchlist.json
 bot_ipc.json
 
-# Test
+# Test (top-level only; signal_v2/tests tracked separately)
 tests/
 tests/*
+!signal_v2/tests/
+!signal_v2/tests/**
 
 # System
 Thumbs.db