P2.2: extract provisioning HTTP boundary

[MichalMatu]

Summary

• move generic provisioning HTTP/JSON plumbing out of manual_config.c into provisioning_http.c/.h
• centralize JSON escaping, typed fields, action responses, profile-list responses and profile error responses
• centralize bounded HTTP form-body reads plus config/LED values, Wi-Fi credentials and profile-index extraction
• keep manual_config.c focused on runtime orchestration, Wi-Fi/scan state and endpoint behavior
• document the P2.2 boundary while leaving P2.3 resource/dependency gates separate

Scope guardrails

This PR does not change endpoint paths or intended JSON schemas. It does not redesign USB NCM, Wi-Fi lifecycle, scan lifecycle, NVS/profile persistence, captive portal behavior or the frontend.

P2.1's strict malformed-form policy remains in force, including rejection of encoded NUL. P2.2 only moves HTTP I/O and presentation responsibilities across a clearer module boundary.

Verification contract

• full ESP32-S2 pio run
• both strict host C policy suites
• frontend checks/build
• cppcheck
• markdownlint