fix: drop broken loan_events FK from loan_disputes migration

[SharifIbrahimDev]

Closes #1191

What does this PR do?

This PR resolves a database migration failure where loan_disputes attempted to create a foreign key referencing loan_events(loan_id). Because loan_events.loan_id lacks a total unique constraint and acts as an append-only event log (and is later renamed to a view via contract_events), PostgreSQL rejects the foreign key.

Description

• Dropped DB-level FK: Removed the REFERENCES loan_events(loan_id) constraint from the loan_disputes schema migration. The schema now relies on application-level validation for loan_id associations, which is already handled seamlessly by the existing robust controllers and indexed events flow.
• Clean Application: This change confirms that the loan_disputes migration will apply cleanly, unaffected by the loan_events -> contract_events rename or the view creation that happens in subsequent migrations.
• Integration Test: Added a dedicated integration test (loanDisputesSchema.test.ts) that asserts an actual dispute record can be cleanly inserted against the real database schema.

[ogazboiz]

this is conflicting with main so it can't merge yet, please rebase: git fetch origin && git rebase origin/main && git push --force-with-lease. once it's clean i'll review the loan_events FK-drop migration, i want to confirm the down() restores it and the loanDisputesSchema test covers the change.

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[ogazboiz]

merging this schema test, it inserts a loan_dispute and validates it against the schema, with a describeIf guard that skips cleanly when no db is available. one note: the title mentions dropping the loan_events FK but the diff is just the test now, looks like the fk-drop landed on main separately and the rebase left only the test here, which is fine. green.

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