feat: Idempotency-Key for POST /emails/send

[patoperpetua]

Summary

• Optional Idempotency-Key on POST /emails/send: absent keeps at-least-once behaviour; present claims a per-tenant Blob ledger before the provider call
• Completed replays return the original SendResponse without a second provider call; in-flight replays return 409 IDEMPOTENCY_IN_PROGRESS
• Persistence: Azure Blob (existing @azure/storage-blob) rather than Table — no new dependency; decision and 24h TTL documented in docs/architecture/send-idempotency.md

Closes #37

Test plan

• pnpm --filter @singleton-sd/post-kit-types test
• pnpm --filter @singleton-sd/post-kit-api test (replay, concurrent in-flight, cross-tenant, no-header, key validation, blob store fakes)
• CI Lint / test / build green
• Human: confirm idempotency container exists (or will be created) on the template storage account before prod use

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📥 Commits

Reviewing files that changed from the base of the PR and between e40f3e1 and 8d09d23.

📒 Files selected for processing (19)

• .env.example
• apps/api/src/config/app-configuration.ts
• apps/api/src/functions/send.idempotency.spec.ts
• apps/api/src/functions/send.ts
• apps/api/src/idempotency/blob-idempotency-store.spec.ts
• apps/api/src/idempotency/blob-idempotency-store.ts
• apps/api/src/idempotency/idempotency-key.spec.ts
• apps/api/src/idempotency/idempotency-key.ts
• apps/api/src/idempotency/idempotency-store.ts
• apps/api/src/idempotency/index.ts
• apps/api/src/idempotency/memory-idempotency-store.spec.ts
• apps/api/src/idempotency/memory-idempotency-store.ts
• docs/architecture/overview.md
• docs/architecture/request-lifecycle.md
• docs/architecture/send-idempotency.md
• docs/operations/troubleshooting.md
• packages/post-kit-types/README.md
• packages/post-kit-types/src/index.spec.ts
• packages/post-kit-types/src/send.ts

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[patoperpetua]

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