fix: reduce rate limit from 10 to 5 RPS to avoid API errors (issue #43)

Reduce RATE_LIMIT_RPS from 10.0 to 5.0 to prevent "초당 거래건수를
초과하였습니다" (EGW00201) errors from KIS API.

Docker logs showed this was the most frequent error (70% of failures),
occurring when multiple stocks are scanned rapidly.

Changes:
- src/config.py: RATE_LIMIT_RPS 10.0 → 5.0
- .env.example: Update default and add explanation comment

Trade-off: Slower API throughput, but more reliable operation.
Can be tuned per deployment via environment variable.

Fixes: #43

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.env.example

@@ -16,8 +16,9 @@ CONFIDENCE_THRESHOLD=80
 # Database
 DB_PATH=data/trade_logs.db
 
-# Rate Limiting
-RATE_LIMIT_RPS=10.0
+# Rate Limiting (requests per second for KIS API)
+# Reduced to 5.0 to avoid "초당 거래건수 초과" errors (EGW00201)
+RATE_LIMIT_RPS=5.0
 
 # Trading Mode (paper / live)
 MODE=paper