Alert persistence tests and Postgres migration

[askmy-stack]

Background

Identified during OSS audit v2 of meridian.

Problem Statement

File-backed alerts untested.

Proposed Solution

Postgres AlertStore.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

src/alerting/persistence.py

---

Suggested labels: priority:medium, difficulty:intermediate, category:testing
Milestone: v2.0-features
OSS Audit: Master Backlog v2