feat: add idempotent upsell trigger logic

[1275321348]

Closes #3

Summary

• Adds an idempotent upsell trigger module for the 5th free call threshold.
• Returns X-Upsell-Prompt: true and threshold metadata when the trigger fires.
• Generates deterministic prompt variants for A/B-test-ready messaging.
• Adds Deno tests for threshold detection, no double-trigger behavior, headers, trigger record shape, and deterministic prompt selection.

Validation

• Ran git diff --check.
• Deno is not installed in this workspace, so deno test tests/upsell.test.ts could not be executed locally.

Notes

The existing migration for upsell_triggers is already present in the repository, so this PR focuses on the expected TypeScript middleware/helper layer and tests.

[1275321348]

Extra implementation note: this PR keeps the upsell trigger logic isolated in a small helper layer and adds coverage for the main behavior: 5th-call threshold detection, duplicate prevention, response header output, trigger record shape, and deterministic prompt variant selection. It should be easy to review or move into a different integration point if needed.