feat(prospecting): add batch delay + fix Celery error_message field

- Add PROSPECTING_BATCH_DELAY_SECONDS config (default 1.0s) — polite
  delay between prospects in batch scans to avoid rate limiting
- Apply delay to all 5 batch API endpoints and all Celery tasks
- Fix Celery tasks: error_message → error_log (matches model field)
- Add batch-scanning.md docs with rate limiting guide, scaling estimates
  for 70k+ URL imports, and pipeline order recommendations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/prospecting/config.py

@@ -26,6 +26,9 @@ class ModuleConfig(BaseSettings):
     # Max concurrent HTTP requests for batch scanning
     max_concurrent_requests: int = 10
 
+    # Delay between prospects in batch scans (seconds) — be polite to target sites
+    batch_delay_seconds: float = 1.0
+
     model_config = {"env_prefix": "PROSPECTING_", "env_file": ".env", "extra": "ignore"}