feat: persist schemaVersion through the event translation pipeline

[CollinsC1O]

Summary

• Add a nullable schemaVersion column (+ index) to the Event model so the blueprint version label computed by translateEvent() is no longer discarded before it reaches the database.
• Persist schemaVersion in both the create and update branches of translateAndPersistEvent's upsert.
• Surface schemaVersion in GET /api/v1/events responses and in the CSV/JSON/NDJSON event exports.

Why

Without a persisted schemaVersion, there's no way to query historical events by the schema version that translated them — e.g. to find events needing re-translation after a blueprint bug fix or contract upgrade.

Test plan

• npx prisma validate / prisma generate succeed with the updated schema
• New tests in persistence.schemaVersion.test.ts confirm schemaVersion reaches db.event.upsert for versioned and unversioned blueprints
• Extended events/route.test.ts to assert schemaVersion is returned by the list API
• Extended events/export/route.test.ts to assert schema_version appears in CSV, JSON, and NDJSON exports
• Full suite run: no new failures introduced (pre-existing unrelated failures in sanitization.test.ts / stats/route.test.ts confirmed present on main too)

Closes #377