feat(routes-f): caesar cipher encode/decode endpoint

[davedumto]

Summary

Add a Caesar cipher endpoint at app/api/routes-f/caesar/route.ts. Encode and decode text with a configurable shift value.

⚠️ Scope Constraint — READ BEFORE STARTING

All files for this task must live inside app/api/routes-f/. This includes:

• The route handler (e.g. app/api/routes-f/<feature>/route.ts)
• Any helpers/utilities (e.g. app/api/routes-f/<feature>/_lib/helpers.ts)
• Any types (e.g. app/api/routes-f/<feature>/_lib/types.ts)
• Any tests (e.g. app/api/routes-f/<feature>/__tests__/route.test.ts)

Do not modify, import from, or add files to lib/, utils/, types/, components/, or anywhere else outside app/api/routes-f/. If you need shared logic, duplicate it inside your subfolder. Keeping everything scoped to this folder is intentional — it keeps these tasks independent and mergeable in any order.

Requirements

• POST /api/routes-f/caesar body { text: string, shift: number, mode: "encode"|"decode" }
• Return { result: string, shift_used: number }
• Preserve case (A -> D, a -> d)
• Preserve non-alphabetic characters unchanged (digits, spaces, punctuation)
• Normalize shift value modulo 26
• Response includes a warning if text is detectably English and shift is unchanged (i.e., shift % 26 === 0)
• Include unit tests, including a round-trip test

Acceptance Criteria

• Encode/decode round-trip is lossless
• Case and non-alpha chars preserved
• Shift modulo normalized
• Tests cover positive, negative, and large shifts
• All files inside app/api/routes-f/caesar/