Implement Request Deduplication Middleware for Idempotent Deployment Creation Endpoints

[temma02]

Description

Build a request deduplication middleware that uses client-supplied idempotency keys to prevent duplicate deployment creation when requests are retried due to network failures or client retries.

Requirements and Context

• Security: Idempotency keys must be scoped per user to prevent cross-tenant collisions
• Testing: Unit and integration tests for the middleware
• Documentation: Document the idempotency key header in the OpenAPI spec

Suggested Execution

Branch: feat/issue-051-deployment-idempotency-middleware

Implement Changes

• Add middleware that reads an Idempotency-Key header on deployment creation routes
• Store processed keys with their responses in a short-lived cache keyed by user + key
• Return the cached response when a duplicate key is received within the window
• Apply the middleware to apps/backend/src/app/api/deployments/route.ts

Test and Commit

• Test that duplicate requests with the same key return the original response
• Test that different keys create separate deployments
• Document the Idempotency-Key header in apps/backend/openapi.yaml

Example Commit Message

feat(deployments): add request deduplication middleware for idempotent creation

- Add Idempotency-Key header handling middleware
- Cache responses per user+key within a time window
- Apply middleware to deployment creation route

Guidelines

• Scope idempotency keys per authenticated user
• Set a reasonable cache TTL (e.g., 24 hours)
• Open a PR against main with tests and OpenAPI updates

[TheShnider]

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[Grace-CODE-D]

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