Add duplicate transaction detection on PDF upload

[7174Andy]

Summary

• Add transaction_exists() method to TransactionRepository that checks for duplicates by matching on (date, amount, description)
• Integrate duplicate detection into the PDF upload flow (UploadDialog) so that re-uploading the same bank statement silently skips already-imported transactions
• Add 5 test cases covering the new repository method

How it works

When a user uploads a Bank of America PDF statement, each parsed transaction is checked against the database before insertion. If a transaction with the same date, amount, and description already exists, it is skipped via continue. This prevents duplicate entries when the same statement is uploaded multiple times.

API changes

TransactionRepository.transaction_exists(transaction: Transaction) -> bool (new)

Checks whether a transaction with matching date, amount, and description already exists in the database. Category is intentionally excluded from the check since it may be updated after initial import.

Design decisions

• Match on (date, amount, description): These three fields uniquely identify a transaction from a bank statement. Category is excluded because it can change via merchant categorization after import.
• Silent skip (no user feedback on skipped count): Keeps the upload flow simple. A future enhancement could report how many duplicates were skipped.
• Per-row check vs. batch: Each transaction is checked individually before insert. For typical statement sizes (30-100 transactions), this is performant enough and keeps the logic straightforward.

Test plan

• test_transaction_exists_returns_true_for_duplicate -- exact match returns True
• test_transaction_exists_returns_false_when_not_found -- empty DB returns False
• test_transaction_exists_different_date -- different date returns False
• test_transaction_exists_different_amount -- different amount returns False
• test_transaction_exists_different_description -- different description returns False
• All 43 existing + new tests pass

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