feat(alerts): notify users when an opportunity crosses a profit threshold

[Flickwire-Agent]

User Story

As an arbitrage trader, I want alerts when an item crosses my profit threshold so that I can act quickly without repeatedly refreshing the app.

Why This Matters

Arbitrage opportunities are time-sensitive. The current app surfaces opportunities on demand, but users still need to poll manually. Threshold alerts turn the app from a dashboard into an active trading assistant.

Acceptance Criteria

• Users can create an alert rule for an item, world/data center target, minimum net spread, and optional minimum recent-sales count.
• Alert rules can be listed, enabled/disabled, edited, and deleted.
• The backend evaluates alert rules whenever opportunity caches refresh.
• Triggered alerts include item name, buy world, sell world/data center, current net spread, recent volume, and generated-at time.
• The first delivery channel can be browser-visible in-app notifications or webhook delivery; the implementation should document the chosen channel.
• Duplicate notifications are suppressed for the same rule while the opportunity remains above threshold.
• Alert evaluation failure does not break opportunity cache refresh.

Useful Context

• The arbitrage cache refresh cadence is currently around 15 minutes.
• There is no user-auth system yet, so scope the first version to locally managed rules plus a backend-compatible data model, or document why server persistence is needed first.
• Future delivery channels may include email, ntfy, Discord, or PWA push.

Suggested Verification

• Add unit or integration coverage for threshold evaluation and duplicate suppression.
• Build API and web packages.
• Manually test creating a rule and seeing a triggered notification in a controlled scenario.