Add CSV import features, test suite improvements, and documentation

[caseybecking]

Features:

• Categories CSV import (API + frontend with drag & drop)
• Transactions CSV import with custom format (API + frontend)
• Smart account/institution creation during transaction import
• Auto-creation of category types and groups during category import
• Duplicate detection for both imports
• Rich description building from multiple CSV fields

Test Improvements:

• Created comprehensive test suite (142 tests total)
• 121/142 tests passing (85% pass rate, up from 62%)
• 100% of model tests passing (63/63)
• 96% of API tests passing (54/56)
• Added input validation to 5 API controllers

Documentation:

• Organized all documentation into /docs directory
• Created comprehensive INDEX.md for navigation
• New root README.md with project overview
• CATEGORIES_IMPORT_FEATURE.md - Complete feature documentation
• TRANSACTIONS_IMPORT_FEATURE.md - Complete feature documentation
• DOCUMENTATION_ORGANIZATION.md - Documentation structure guide

Bug Fixes:

• Fixed account creation 400 error on frontend
• Fixed validation to allow optional fields (None values)
• Added missing default fields to account creation form

Files Modified:

• api/account/controllers.py - Added validation
• api/categories/controllers.py - Added CSV import endpoint and validation
• api/institution/controllers.py - Added validation
• api/institution_account/controllers.py - Fixed validation logic
• api/transaction/controllers.py - Rewrote CSV import with new format
• app/categories/controllers.py - Added import route
• app/static/js/institution_account/institution_account.js - Added default fields
• app/templates/categories/index.html - Added import button
• app/templates/transactions/import.html - New drag & drop interface
• app/templates/transactions/index.html - Updated import button

Files Created:

• app/static/js/categories/import.js - Categories upload handler
• app/static/js/transactions/import.js - Transactions upload handler
• app/templates/categories/import.html - Categories import UI
• tests/ - Complete test suite (142 tests)
• docs/ - All documentation moved here
• .coveragerc - Coverage configuration
• pytest.ini - Pytest configuration