feat: bulk import validation & preview improvements (#115)

[sungdark]

Summary

Implements per-row validation with warnings and auto-corrections for the bulk import preview endpoint, closing issue #115.

What was built

Backend — expense_import.py (+248 lines)

• `validate_import_rows()` — validates a list of normalized transaction rows
• `RowValidationResult` dataclass: row_index, is_valid, has_errors, warnings, corrections, original
• `BulkImportValidationResult` dataclass: totals + per-row results + corrected preview
• Validation checks per row:
  • Date: invalid/missing = error; future date = error in strict mode, warning otherwise; very old (< 1990) = warning
  • Amount: missing/invalid = error; zero = warning (error in strict mode); negative → auto-corrected to abs(); > 100k = large-amount warning
  • Description: missing/empty → auto-corrected to "Unknown" with warning
  • Currency: missing → auto-corrected to "USD" with warning
  • expense_type: unknown → auto-corrected to "EXPENSE" with warning

Backend — expenses.py (preview endpoint enhanced)

• `POST /expenses/import/preview` now returns:
  • `valid_rows`, `warning_rows`, `error_rows`, `ready_to_import`
  • `row_validations`: per-row list of {row_index, is_valid, has_errors, warnings, corrections}
  • Duplicate detection results merged into row warnings
  • Auto-corrections applied to the `transactions` list in the response
• New query param `?strict=true` to treat warnings as blocking errors

Tests — test_expenses.py (+408 lines)

• 15 unit tests for the validation service (all passing)
• 12 integration tests for the enhanced /import/preview endpoint

PR Claim

/claim #115

[sungdark]

/claim #115

Implementation Summary

What was built

Backend — `expense_import.py` (+248 lines)

• `validate_import_rows()` validates a list of normalized transaction rows
• Per-row checks: date validity, amount validity, zero-amount, future/old dates, large amounts, missing description/currency, unknown expense_type
• Auto-corrections: missing description → "Unknown", missing currency → "USD", unknown expense_type → "EXPENSE", negative amounts → abs()

Backend — `expenses.py` (preview endpoint enhanced)

• `POST /expenses/import/preview` now returns:
  • `valid_rows`, `warning_rows`, `error_rows`, `ready_to_import`
  • `row_validations`: list of {row_index, is_valid, has_errors, warnings, corrections}
  • Auto-corrections applied to `transactions` list
  • `?strict=true` treats zero amounts and future dates as errors

Tests (+408 lines)

• 15 unit tests for validation service — all passing
• 12 integration tests for enhanced preview endpoint

/close #115