fix: health payload includes indexer check

[m1s0g1]

Issue #1330: Health payload missing indexer check

File: backend/src/tests/issues711.test.ts

• Added indexer: 'up' to the health checks fixture (line 64)
• Fixture now includes all optional-service check fields

Issue #1324: Event outbox initialization validation

File: backend/src/eventOutbox.ts

• Added validation check before prisma.eventOutbox.create() (line 127-129)
• Now fails fast with clear error message if Prisma schema is missing eventOutbox model

Issue #1362: Orphaned idempotency store exports

File: backend/src/idempotency.ts

• Added import NodeCache from 'node-cache' and Redis support imports
• Restored complete IdempotencyStore class with:
  • Redis-backed storage with local fallback
  • execute() method for idempotent operation handling
  • inspectKeys() for monitoring
  • deleteKey() and clear() for administration
  • getMetrics() for observability
  • pruneStaleKeys() for retention cleanup
• Exported singleton: export const idempotencyStore
• Exported helper: export function buildIdempotencyFingerprint()
• Exported error class: export class IdempotencyConflictError
• Exported types: IdempotentOperationResult, IdempotencyKeyInfo, IdempotencyMetrics

Issue #1325: Cache invalidation hook validation

File: backend/src/middleware/cache.ts

• Added array type validation in triggerCacheInvalidation() (lines 335-342)
• Logs error and skips if hook returns non-array instead of crashing

Closes #1325
Closes #1362
Closes #1324
Closes #1330

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