feat: add routes-f update endpoint with optimistic concurrency

[Jay-Peter-Egemasi]

Description

Closes #299

Changes proposed

What were you told to do?

Backend: Add routes-f update endpoint with optimistic concurrency (#299)

What did you do?

• Added optional etag property to the RoutesFRecord interface (lib/routes-f/types.ts).
• Created getRoutesFRecordById and updateRoutesFRecord methods in the mock store (lib/routes-f/store.ts), managing ETag matching logic and ETag assignments for updates.
• Added PATCH and GET handlers for /api/routes-f/items/[id] (app/api/routes-f/items/[id]/route.ts). The PATCH endpoint verifies the If-Match header and returns a 428 error if it is missing or 412 if the ETag mismatches.
• Authored test cases covering these concurrency flows (app/api/routes-f/__tests__/items.test.ts).

Check List (Check all the applicable boxes)

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• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title and description of the PR is clear and explains the approach.
• I am making a pull request against the main branch (left side).
• My commit messages styles matches our requested structure.
• My code additions will fail neither code linting checks nor unit test.
• I am only making changes to files I was requested to.

Screenshots/Videos

N/A - This is a purely backend feature with tests validating the requests and responses under the hood.

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