Add multipart document uploads for the 16 resources that have the route

First item on the GOALS.md roadmap. `upload_{x}(id, files, is_public)`
attaches files through `/{resource}/{id}/upload` and returns the
refreshed record. Verified against upstream that all 16 controllers
answer with `itemResponse($x->fresh())`.

Three details that look wrong but are not, each checked in the upstream
source rather than taken from the published spec:

- The request is a POST carrying `_method=PUT`. The route is registered
  PUT, but PHP does not parse a multipart body on a real PUT, so the
  files never arrive. Laravel's kernel calls
  `enableHttpMethodParameterOverride()`, turning the spoofed method back
  into a PUT before routing. The spec documents POST with `_method=POST`,
  which would 405 against these routes.
- `is_public` is sent as 1/0. Laravel's `boolean` rule accepts only
  true, false, 0, 1, '0' and '1' under strict comparison, so "true"
  fails validation.
- Uploads are never retried. An upload adds a document, so it is not
  idempotent whatever its verb, and a replay after a lost response would
  attach the file twice. This falls out of reqwest's multipart body being
  un-cloneable, which nothing in the types enforces, so a test pins it
  for 500, 503 and 429.

An upload with no files, a nameless file or an unparseable MIME type is
refused locally (`Error::InvalidUpload`), since the server answers an
empty upload with a success that changed nothing.

Corrects two earlier claims:

- GOALS.md and COVERAGE.md said uploads would make `create_document`
  possible. They do not: `DocumentController::store` is an empty stub,
  and an upload is the only way a document is created. There is no
  `create_document`.
- The documented method count (265) had been stale since the typed
  `statics`/`search`/`health_check` commit, which added three `_as`
  methods without updating it. Recounted from the source: 284 = 210 +
  18 + 12 + 16 uploads + 28 hand-written.

Also fixes a README example, `let statics: Value = client.statics()`,
that stopped compiling when `statics` lost its type parameter.

`edocument/upload` is left out: it imports an expense from an
e-document rather than attaching a file, so it moves to the
import/export roadmap item.

Enabling `multipart` adds mime, mime_guess and unicase; MSRV stays 1.88,
re-verified by running the suite on it. 101 tests, up from 91.

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

README.md

@@ -100,7 +100,7 @@ IN_API_TOKEN=... cargo run --example list_clients
 
 ## Coverage
 
-Every one of the API's 34 resource groups, plus users — 265 methods. Each
+Every one of the API's 34 resource groups, plus users — 284 methods. Each
 resource has `list`, `list_all`, `get`, `create`, `update`, `delete`, and
 (where the API offers one) `bulk`:
 
@@ -158,15 +158,33 @@ let pdf: Vec<u8> = client.download_invoice("abc123").await?;
 let merged: Vec<u8> = client.download_invoices(&["abc123", "def456"]).await?;
 ```
 
-Reports return their raw export body (CSV for most of them), and the
-schemaless endpoints decode into any type you choose:
+Documents are attached through the record they belong to, since that is the
+only way the API creates one. Sixteen resources support it, as
+`upload_client`, `upload_invoice`, `upload_expense`, and so on:
+
+```rust
+use invoiceninjars::FileUpload;
+
+let receipt = FileUpload::new("receipt.pdf", std::fs::read("receipt.pdf")?)
+    .mime("application/pdf");
+
+// `Some(true)` makes the document visible to the client in the portal.
+let expense = client.upload_expense("abc123", vec![receipt], Some(false)).await?;
+```
+
+Files are held in memory while uploading, and an upload is never retried,
+since a replay after a lost response would attach the file twice.
+
+Reports return their raw export body (CSV for most of them). `statics`,
+`search`, and `health_check` return typed results; `chart` has no common
+response shape, so it decodes into any type you choose:
 
 ```rust
 use serde_json::{json, Value};
 
 let csv: Vec<u8> = client.report("invoices", &json!({"date_range": "this_year"})).await?;
 let totals: Value = client.chart("totals_v2", &json!({})).await?;
-let statics: Value = client.statics().await?;
+let statics = client.statics().await?;
 ```
 
 Add more resources by adding a model under `src/models/` and a