Add Outline API client library foundation

Implements a first vertical slice of the Outline RPC API (auth, documents,
collections, users) covering the structural patterns used across the whole
API: single object, paginated list, tree, create/update, delete, search.
Chosen as the basis for future CLI and GUI clients built on top of this crate.

- Handwritten client (not codegen) against the vendored OpenAPI spec, since
  Outline's API is uniformly POST /api/<resource>.<action> with JSON bodies
  and inline/anonymous schemas that generators handle poorly
- Async (reqwest + tokio) Client, cheaply cloneable, no &mut self methods
- thiserror-based Error with ErrorKind classification and boxed API error
  context; forward-compatible models (unknown fields ignored, unknown
  string-enum values preserved via a catch-all variant) since the API is
  unversioned and self-hosted instances vary in age
- Pagination via Page<T>/Paginator with next_page/collect_all/into_stream
- wiremock-based test suite plus a spec-coverage test guarding against typos
  in RPC method names

src/models/collection.rs

@@ -0,0 +1,66 @@
+use serde::{Deserialize, Serialize};
+
+use super::common::{Id, Permission, Timestamp, UnknownVariant};
+use super::user::User;
+
+/// A collection: a top-level grouping of documents in Outline.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+#[non_exhaustive]
+pub struct Collection {
+    /// Unique identifier for the collection.
+    pub id: Id,
+    /// The relative URL path at which the collection can be accessed.
+    #[serde(default)]
+    pub url: Option<String>,
+    /// A short unique identifier that can be used in place of the UUID.
+    #[serde(default)]
+    pub url_id: Option<String>,
+    /// The name of the collection.
+    pub name: String,
+    /// A description of the collection, may contain markdown formatting.
+    #[serde(default)]
+    pub description: Option<String>,
+    /// The position of the collection in the sidebar.
+    #[serde(default)]
+    pub index: Option<String>,
+    /// A color representing the collection, in `#RRGGBB` format.
+    #[serde(default)]
+    pub color: Option<String>,
+    /// An icon name or emoji associated with the collection.
+    #[serde(default)]
+    pub icon: Option<String>,
+    /// The sharing permission level for this collection.
+    #[serde(default)]
+    pub permission: Option<Permission>,
+    /// Whether public document sharing is enabled in this collection.
+    #[serde(default)]
+    pub sharing: bool,
+    /// Whether commenting is enabled in this collection.
+    #[serde(default)]
+    pub commenting: Option<bool>,
+    /// The date and time this collection was created.
+    #[serde(default)]
+    pub created_at: Option<Timestamp>,
+    /// The date and time this collection was last changed.
+    #[serde(default)]
+    pub updated_at: Option<Timestamp>,
+    /// The date and time this collection was archived, if applicable.
+    #[serde(default)]
+    pub archived_at: Option<Timestamp>,
+    /// The user who archived this collection, if applicable.
+    #[serde(default)]
+    pub archived_by: Option<User>,
+}
+
+/// The status a collection may be filtered by in `collections.list`.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+#[non_exhaustive]
+pub enum CollectionStatus {
+    /// The collection has been archived.
+    Archived,
+    /// A value not recognized by this version of the crate.
+    #[serde(untagged)]
+    Unknown(UnknownVariant),
+}