P5-2: Add unique constraint on idempotency_key

[tcheeric]

Description:
Enforce idempotency key uniqueness in the database.

Changes required:

• Add unique constraint on idempotency_key column.
• Ensure migration handles existing data.

Acceptance criteria:

• Database enforces uniqueness for idempotency_key.
• Duplicate keys are rejected by constraint.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-5-task-2
Dependencies: P5-1