refactor(logging): Add optional logging support throughout codebase

Add centralized logging utility and optional logger parameters to all
core functions for better observability and debugging capabilities.

New modules:
- utils/logging.py: Centralized logger configuration with console
  and optional file handlers

Enhanced features:
- Added optional logger parameter to all extractor_core functions
- Added logger support to extractor, excel_converter, and auth modules
- Functions remain silent when logger=None (backward compatible)
- Improved environment variable validation in test files

Documentation:
- Added discrete_material_plan_extractor_core.md with complete API
  reference and usage patterns

Benefits:
- Consistent logging format across all components
- Optional debug output for troubleshooting
- No breaking changes - fully backward compatible
- Better error messages and validation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_extractor_real.py

@@ -86,12 +86,20 @@ try:
     # Setup browser and extract data
     print("\n[3/6] Launching browser and logging in...")
     with sync_playwright() as playwright:
+        # Get required environment variables (with validation)
+        username = os.getenv("ERP_USERNAME")
+        password = os.getenv("ERP_PASSWORD")
+        base_url = os.getenv("ERP_URL")
+        
+        if not username or not password or not base_url:
+            raise ValueError("Missing required environment variables: ERP_USERNAME, ERP_PASSWORD, ERP_URL")
+        
         # Login
         browser, context, page, main_frame = login(
             playwright=playwright,
-            username=os.getenv("ERP_USERNAME"),
-            password=os.getenv("ERP_PASSWORD"),
-            url=get_login_url(os.getenv("ERP_URL")),
+            username=username,
+            password=password,
+            url=get_login_url(base_url),
             headless=os.getenv("ERP_HEADLESS", "true").lower() == "true",
             ignore_https_errors=os.getenv("ERP_IGNORE_HTTPS_ERRORS", "true").lower() == "true",
             verbose=True,