feat: Rebuild complete CSV import system for legacy data migration

PROBLEM SOLVED:
- Completely removed broken import functionality
- Built new robust, modular CSV import system from scratch
- Provides reliable data migration path for legacy .sc files

NEW IMPORT SYSTEM FEATURES:
✅ Modular CSV parsers for all 5 tables (ROLODEX, PHONE, FILES, LEDGER, QDROS)
✅ RESTful API endpoints with background processing (/api/admin/import/*)
✅ Admin web interface at /admin/import for file uploads
✅ Comprehensive validation and error handling
✅ Real-time progress tracking and status monitoring
✅ Detailed logging with import session tracking
✅ Transaction rollback on failures
✅ Batch import with dependency ordering
✅ Foreign key validation and duplicate detection

TECHNICAL IMPLEMENTATION:
- Clean /app/import_export/ module structure with base classes
- Enhanced logging system with import-specific logs
- Background task processing with FastAPI BackgroundTasks
- Auto-detection of CSV delimiters and encoding
- Field validation with proper data type conversion
- Admin authentication integration
- Console logging for debugging support

IMPORT WORKFLOW:
1. Admin selects table type and uploads CSV file
2. System validates headers and data structure
3. Background processing with real-time status updates
4. Detailed error reporting and success metrics
5. Import logs stored in logs/imports/ directory

SUPPORTED TABLES:
- ROLODEX (contacts/people) - 19 fields, requires: id, last
- PHONE (phone numbers) - 3 fields, requires: rolodex_id, phone
- FILES (case files) - 29 fields, requires: file_no, id, empl_num, file_type, opened, status, rate_per_hour
- LEDGER (transactions) - 12 fields, requires: file_no, date, t_code, t_type, empl_num, amount
- QDROS (documents) - 31 fields, requires: file_no

REMOVED FILES:
- app/api/unified_import_api.py
- app/services/unified_import.py
- app/api/flexible.py
- app/models/flexible.py
- templates/unified_import.html
- templates/flexible.html
- static/js/flexible.js
- All legacy import routes and references

TESTING COMPLETED:
✅ Schema validation for all table types
✅ CSV header validation
✅ Single file import functionality
✅ Multi-table dependency validation
✅ Error handling and logging
✅ API endpoint integration

READY FOR PRODUCTION: System tested and validated with sample data.
Administrators can now reliably import CSV files converted from legacy .sc files.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app/import_export/ledger_importer.py

@@ -0,0 +1,113 @@
+"""
+LEDGER CSV Importer
+"""
+from typing import Dict, List, Any
+from datetime import date
+from sqlalchemy.orm import Session
+
+from .base import BaseCSVImporter, ImportValidationError
+from app.models.ledger import Ledger
+from app.models.files import File
+
+
+class LedgerCSVImporter(BaseCSVImporter):
+    """CSV importer for LEDGER table"""
+    
+    @property
+    def table_name(self) -> str:
+        return "ledger"
+        
+    @property
+    def required_fields(self) -> List[str]:
+        return ["file_no", "date", "t_code", "t_type", "empl_num", "amount"]
+        
+    @property
+    def field_mapping(self) -> Dict[str, str]:
+        """Map CSV headers to database field names"""
+        return {
+            "file_no": "file_no",
+            "item_no": "item_no",
+            "date": "date",
+            "t_code": "t_code",
+            "t_type": "t_type",
+            "t_type_l": "t_type_l",
+            "empl_num": "empl_num",
+            "quantity": "quantity",
+            "rate": "rate",
+            "amount": "amount",
+            "billed": "billed",
+            "note": "note"
+        }
+        
+    def create_model_instance(self, row_data: Dict[str, Any]) -> Ledger:
+        """Create a Ledger instance from processed row data"""
+        
+        # Validate required fields
+        required_checks = [
+            ("file_no", "File number"),
+            ("date", "Date"),
+            ("t_code", "Transaction code"),
+            ("t_type", "Transaction type"),
+            ("empl_num", "Employee number"),
+            ("amount", "Amount")
+        ]
+        
+        for field, display_name in required_checks:
+            if not row_data.get(field):
+                raise ImportValidationError(f"{display_name} is required")
+                
+        # Validate foreign key exists (file number)
+        file_exists = self.db_session.query(File).filter_by(file_no=row_data["file_no"]).first()
+        if not file_exists:
+            raise ImportValidationError(f"File number '{row_data['file_no']}' does not exist")
+            
+        # Parse date
+        try:
+            transaction_date = self.parse_date(row_data["date"])
+        except ValueError as e:
+            raise ImportValidationError(f"Invalid date: {e}")
+            
+        # Parse numeric fields
+        try:
+            item_no = 1  # Default
+            if row_data.get("item_no"):
+                item_no = self.parse_int(row_data["item_no"])
+                if item_no < 1:
+                    raise ImportValidationError("Item number must be positive")
+        except ValueError as e:
+            raise ImportValidationError(f"Invalid item number: {e}")
+            
+        try:
+            quantity = self.parse_float(row_data.get("quantity", "0"))
+            rate = self.parse_float(row_data.get("rate", "0"))
+            amount = self.parse_float(row_data["amount"])
+        except ValueError as e:
+            raise ImportValidationError(f"Invalid numeric value: {e}")
+            
+        # Validate transaction code and type
+        t_code = self.normalize_string(row_data["t_code"], 10)
+        t_type = self.normalize_string(row_data["t_type"], 1)
+        t_type_l = self.normalize_string(row_data.get("t_type_l", ""), 1)
+        
+        # Validate billed field (Y/N)
+        billed = row_data.get("billed", "N").strip().upper()
+        if billed not in ["Y", "N", ""]:
+            billed = "N"  # Default to N if invalid
+            
+        # Create instance
+        ledger = Ledger(
+            file_no=self.normalize_string(row_data["file_no"], 45),
+            item_no=item_no,
+            date=transaction_date,
+            t_code=t_code,
+            t_type=t_type,
+            t_type_l=t_type_l,
+            empl_num=self.normalize_string(row_data["empl_num"], 10),
+            quantity=quantity,
+            rate=rate,
+            amount=amount,
+            billed=billed,
+            note=row_data.get("note", "")  # Text field
+        )
+        
+        return ledger