Wrap the e-invoicing and integration routes

Closes the last roadmap item, and with it the roadmap: every route in
`routes/api.php` that an API-token client can reach is now wrapped. 33
methods — 17 e-invoicing, 4 QuickBooks, 4 Stripe, 6 calendar, and one each
for Yodlee and GoCardless.

Most return `serde_json::Value`, which is deliberate and is the crate's
largest untyped surface. These replies are pass-throughs from Storecove,
Stripe's gateway driver, Yodlee or GoCardless, or arrays assembled for
Invoice Ninja's own React client. None has a documented schema, several
differ by provider, and they move on the provider's release schedule rather
than Invoice Ninja's — so the rule that keeps `chart` generic applies here
with more force, not less. `client_peppol_discovery` set the precedent.

Three upstream behaviours needed accommodating, each now pinned by a test:

- `validate_einvoice` answers 422 when validation finds problems, so an
  ordinary "this invoice is not Peppol-ready" result arrives as `Error::Api`
  rather than an `Ok` carrying the failures. The per-entity detail is not
  reachable through the typed error. Documented rather than worked around.
- `einvoice/peppol/setup` has two success shapes: a `corppass_url` when the
  connection needs a further browser step, or a bare 204 when it completed.
  Hence `post_unwrapped_opt` and `Option<Value>`, since decoding the 204
  would otherwise be a spurious error.
- `einvoice/peppol/remove_additional_legal_identifier` is a DELETE carrying
  a body — the only one in the crate — and `einvoice/peppol/update` a PUT
  with one, hence `send_unwrapped`.

The Peppol handlers also return `data_get($response, 'message')` as the
entire error body, which may be a bare string rather than
`{"message": …, "errors": {…}}`. `Error::Api` then falls back to an excerpt
with an empty `errors` map; the docs say so on the impl block.

Two things in this block stay unwrapped because they cannot be wrapped. The
OAuth legs of the QuickBooks and calendar flows sit outside the `api/v1`
group, take no token, and answer with redirects, so connecting an account
remains a browser flow — these methods operate a connection that already
exists. And `POST /quickbooks/settings` is a registered route whose
controller has no `settings` method, so it raises `BadMethodCallException`
and 500s; there is nothing to wrap until upstream adds the handler. Both are
now entries in `COVERAGE.md`'s gap list rather than silent omissions.

`GOALS.md` keeps the roadmap rather than deleting it: several entries record
what an item's original reasoning got wrong, which outlasts the fact that it
shipped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,21 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- E-invoicing, seventeen methods: `validate_einvoice`,
+  `einvoice_configurations`, `einvoice_quota`, `einvoice_health_check`,
+  `einvoice_delivery_map`, `update_einvoice_token`, and eleven `peppol_*`.
+  Three behave unlike the rest of the API, and the docs on each say so: a
+  failing `validate_einvoice` is a `422`, so an ordinary "not Peppol-ready"
+  result arrives as `Error::Api`; the Peppol handlers return the error
+  message as the whole body rather than `{"message": …}`, so `Error::Api`'s
+  `errors` map is empty for them; and `peppol_setup` has two success shapes,
+  hence `Option<Value>`. `peppol_remove_legal_identifier` is the crate's
+  only `DELETE` with a request body.
+- Integrations, sixteen methods: four `quickbooks_*`, four `stripe_*` (all
+  password-protected), six `calendar_connection` methods, plus
+  `yodlee_account_status` and `nordigen_institutions`. These operate a
+  connection that already exists — establishing one is an OAuth browser flow
+  whose routes take no API token and cannot be wrapped.
 - Import and export, five methods: `export`, `preimport`, `import`,
   `import_json`, and `upload_edocument`. A CSV import is two calls —
   `preimport` uploads the files as `files[<entity_type>]` and returns the