fix(backend): add IdempotencyKey model, regenerate contract snapshots, generate Prisma client before build

[francisdouglas-ux]

Summary

• Idempotency contract model must expose idempotencyKey field in Prisma types #1310 – adds the IdempotencyKey Prisma model used by src/idempotency.ts (unique on keyId + tenantId, indexed on expiresAt) with an additive migration.
• Prisma client regeneration must run before contract type-checking in CI #1311 – adds a prebuild script (npm run prisma:generate) so the Prisma client is regenerated before tsc wherever npm run build runs, including CI.
• API contract snapshots must be regenerated when response envelope fields change #1312 / Ready response contract schema must include the indexer check field #1314 – regenerated schema-snapshots/ via npm run snapshots:write: get-_ready.json now includes dependencies.indexer (boolean) and get-_health.json includes checks.indexer. npm run snapshots:check passes.

Closes #1310
Closes #1311
Closes #1312
Closes #1314

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