feat(routes-f): n-gram extractor for text analysis (reissue)

[davedumto]

Summary

Add an n-gram extractor at app/api/routes-f/n-grams/route.ts. Extract n-grams of any size with frequency counts.

Requirements

• POST body { text, n: number, level?: word|char, lowercase?, top? }
• Return { ngrams: [{ gram, count }], total_grams, unique_grams }
• Default level word, top 50
• Validate n in [1, 10]
• Cap input at 1MB
• Tests cover word vs char modes, varied n, frequency ordering

Scope Constraint - READ BEFORE STARTING

All files for this task must live inside app/api/routes-f/. This includes the route handler, helpers/utilities, types, and tests. 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.