Add threshold alerts and oracle relay example

[ALIPHATICHYD]

Summary:

• Extracted threshold alert matching and webhook delivery into a small shared alerts module.
• Added a minimal Soroban oracle contract example plus a Node relay that polls Lens and pushes prices periodically.
• Documented the relay example and added a convenience script.

Closes #64
Closes #65

[drips-wave]

@ALIPHATICHYD Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Miracle656]

Both halves are well-executed.

Threshold alert extraction

• crossesThreshold is edge-triggered (prev < threshold && current ≥ threshold for above), not level-triggered — that's the right semantics so a price sitting above the threshold doesn't spam alerts every poll.
• deliverJsonWithRetries correctly differentiates 4xx (no retry — won't ever succeed) from network failures and 5xx (retry up to 3 attempts with 2^(attempt−1)·1s backoff). 10 s timeout per attempt.
• HMAC over JSON.stringify(payload) with X-Lens-Signature matches the existing Lens convention.
• webhookDispatcher.ts going −70/+4 confirms the extraction is clean — no logic forked.

Oracle relay example

• Contract (set_price / get_price + DataKey::Price in instance storage) is the minimum-viable shape; perfect for a docs example. (For production you'd want i128 + scale instead of String, but starting simple is the right call here.)
• relay.ts is a complete reference: configurable env, polls Lens REST /price/{a}/{b}, simulates → assembles → signs → submits via Soroban RPC. The --once flag in the help text is a nice touch for cron-style use.
• README explains the wiring.

Merging. Closes #64, #65.