US-X.4implemented Unique transaction hashes prevent duplicates
As a user who might accidentally import the same CSV twice I want every transaction to have a unique SHA256 hash based on its content so that the pipeline detects and rejects duplicate imports.
Acceptance Criteria
- Each `Transaction` has a `get_hash()` method producing a deterministic hash.
- Importing the same CSV twice does not create duplicate journal entries.
- Duplicate detection works across both CSV transactions and receipt-linked transactions.
No DAG diagram: Cross-cutting constraint — hash uniqueness is verified within US-3.14 (duplicate blocked) path; this story captures the broader requirement.
Demo video coming soon