Convert an e-invoice between formats

Auto-detects and parses the source e-invoice, then regenerates it in the requested target format (`zugferd`, `facturx`, `xrechnung`). XRechnung → ZUGFeRD is loss-free (CIUS → EN 16931); the reverse only succeeds when the XRechnung obligations (BT-10, BR-DE-*) are met — otherwise a 422 with the offending rule codes is returned. `zugferd` and `facturx` are the same hybrid PDF/A-3 standard — the seller country picks the flavour (FR/BE → Factur-X, otherwise ZUGFeRD), so both requests yield a PDF with the embedded CII XML. Any fields not carried across are listed in `conversionWarnings`. `warnings` carries every warning of the target generator and of its conformance self-check, in the same shape as `/generate` — among them `SELF_CHECK_INCOMPLETE` when a rulebook could not be applied: the document was then not fully checked against it. The target is written in ZUGFeRD `2.4` / Factur-X `1.08` — the same default as `/generate` — and `appliedFormatOptions` names the version and profile actually written. A profile the target generator cannot write, or one that conflicts with the source's `countrySpecific.facturxProfile`, returns a 400 with a `code`. Requires BOTH the source `:parse` and the target `:create` entitlement; usage is billed once against the target `:create` key (like generate).