feat(routes-f): content-type header parser

[davedumto]

Summary

Parse a Content-Type header into type, subtype, and parameters.

Requirements

• POST { header }
• Return { type, subtype, parameters: { charset?, boundary?, ... } }
• Handle quoted parameter values
• Reject malformed headers with 400
• Tests cover charset, multipart boundary, vendor types

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.