feat: require reason and audit trail for verification override

[Vvictor-commits]

Closes #548

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Summary

Closes the oracle-manipulation accountability gap in admin_override_verification. Every override now requires a non-empty justification, appends an immutable audit-trail record, and emits a dedicated event for off-chain monitors.

Changes

audit_trail.rs

• Added OracleVerificationOverride variant to AuditAction enum

events.rs

• Added AdminOverrideEvent struct (market_id, admin, old_result, new_result, reason, timestamp)
• Added EventEmitter::emit_admin_override with topic key adm_ovrd

lib.rs

• Replaced the always-failing stub with a real implementation:
  1. require_primary_admin — auth before any storage write
  2. Empty reason → Err(InvalidInput) before touching storage
  3. Loads market, captures old_result, sets oracle_result + state = Resolved, saves
  4. Appends audit record with old_result/new_result/reason
  5. Emits AdminOverrideEvent

override_audit_tests.rs (new, #[cfg(test)])

• 6 self-contained tests, no external test helpers

Acceptance Criteria

Criterion	Test
Override rejects empty reason	test_override_rejects_empty_reason
Audit record written with correct action/actor/reason	test_override_appends_audit_record
verify_audit_integrity passes after override	test_override_preserves_audit_integrity
Market state = Resolved, oracle_result set	test_override_resolves_market
Non-admin rejected	test_override_rejects_non_admin
No partial state on auth failure	test_override_no_partial_state_on_auth_failure

Testing

cargo test -p predictify-hybrid -- override_audit

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