P3-5: Implement DB-backed idempotency via idempotency_key

[tcheeric]

Description:
Use idempotency_key column to deduplicate webhook processing.

Changes required:

• Add idempotency store logic in dispatcher.
• Handle duplicates with WebhookDuplicateException (200).

Acceptance criteria:

• Duplicate webhook payloads are treated as idempotent success.
• Idempotency is persisted to the database.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-3-task-5
Dependencies: P3-4