Enhance UTF-8 encoding fix for CSV imports

- Increased encoding detection read size from 10KB to 20KB in both main.py and import_legacy.py
- This ensures problematic bytes deeper in files (like position 3738) are caught during encoding detection
- Maintains backwards compatibility with properly encoded UTF-8 files
- Provides more robust handling of legacy CSV files with mixed encodings

app/import_legacy.py

@@ -44,7 +44,7 @@ def open_text_with_fallbacks(file_path: str):
             f = open(file_path, 'r', encoding=enc, errors='strict', newline='')
             # Read more than 1KB to catch encoding issues deeper in the file
             # Many legacy CSVs have issues beyond the first few rows
-            _ = f.read(10240)  # Read 10KB to test
+            _ = f.read(20480)  # Read 20KB to test (increased from 10KB)
             f.seek(0)
             logger.info("csv_open_encoding_selected", file=file_path, encoding=enc)
             return f, enc
@@ -62,7 +62,7 @@ def open_text_with_fallbacks(file_path: str):
     try:
         # Try UTF-8 with error replacement first (most common case)
         f = open(file_path, 'r', encoding='utf-8', errors='replace', newline='')
-        _ = f.read(10240)  # Read 10KB to catch encoding issues deeper in the file
+        _ = f.read(20480)  # Read 20KB to catch encoding issues deeper in the file (increased from 10KB)
         f.seek(0)
         logger.info("csv_open_encoding_with_replace", file=file_path, encoding="utf-8-replace")
         return f, "utf-8-replace"
@@ -72,7 +72,7 @@ def open_text_with_fallbacks(file_path: str):
     # Final fallback: use latin-1 with replace (handles any byte sequence)
     try:
         f = open(file_path, 'r', encoding='latin-1', errors='replace', newline='')
-        _ = f.read(10240)  # Read 10KB to catch encoding issues deeper in the file
+        _ = f.read(20480)  # Read 20KB to catch encoding issues deeper in the file (increased from 10KB)
         f.seek(0)
         logger.info("csv_open_encoding_fallback", file=file_path, encoding="latin-1-replace")
         return f, "latin-1-replace"