Add retry with backoff, and narrow the untyped responses

Retry:
- `RetryPolicy` (3 retries, full-jitter exponential backoff) applied in a
  new `execute` dispatch chokepoint, configurable via
  `with_retry_policy`. `Retry-After` is honoured, and one longer than
  `max_delay` is surfaced rather than slept through.
- What may be replayed depends on the method, not just the status. A 429
  means the server refused the request without processing it, so any
  method is safe. A 5xx or timeout gives no such guarantee, so only
  idempotent methods are retried: this is a billing API, and replaying a
  POST the server had already applied would duplicate an invoice,
  payment, or email.

Response types:
- `statics`, `search`, and `health_check` return `Statics`,
  `SearchResults`, and `HealthCheck` instead of being generic.
  `HealthCheck` covers both deployments — hosted returns a bare message
  rather than a report — and `SearchResults` covers both the
  Elasticsearch and non-Elasticsearch bucket sets.
- Each keeps the `extra` convention, and `statics_as` / `search_as` /
  `health_check_as` remain for pinning shapes left untyped. Nested config
  blobs (gateways, templates, extensions) stay `Value`, same reasoning as
  `Company::settings`.
- `send_email` previously decoded the response as a bare body, but the
  controller returns `itemResponse(...)` — a `data`-wrapped entity. It now
  unwraps, so `T` is the entity. Still generic, since the endpoint serves
  invoices, quotes, credits, and purchase orders.
- `chart` stays generic on purpose: the eleven charts share no response
  shape.
- Adds `id_as_string`, since `/statics` returns `payment_types.id` as a
  string and the other tables return numbers.

Adds CHANGELOG.md (Keep a Changelog), and a CLAUDE.md note to keep it
current. 17 new tests, 43 in client_requests.rs total.

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

src/models/mod.rs

@@ -12,6 +12,7 @@ mod document;
 mod expense;
 mod expense_category;
 mod group_setting;
+mod health;
 mod invoice;
 mod location;
 mod payment;
@@ -21,6 +22,8 @@ mod project;
 mod purchase_order;
 mod recurring_expense;
 mod recurring_invoice;
+mod search;
+mod statics;
 mod subscription;
 mod system_log;
 mod tag;
@@ -46,6 +49,7 @@ pub use document::Document;
 pub use expense::Expense;
 pub use expense_category::ExpenseCategory;
 pub use group_setting::GroupSetting;
+pub use health::{HealthCheck, PhpVersion};
 pub use invoice::{Credit, Invoice, InvoiceItem, Quote};
 pub use location::Location;
 pub use payment::{Payment, PaymentInvoice};
@@ -55,6 +59,8 @@ pub use project::Project;
 pub use purchase_order::PurchaseOrder;
 pub use recurring_expense::RecurringExpense;
 pub use recurring_invoice::{RecurringInvoice, RecurringQuote};
+pub use search::{SearchHit, SearchResults};
+pub use statics::{Currency, DateFormat, StaticItem, Statics};
 pub use subscription::Subscription;
 pub use system_log::SystemLog;
 pub use tag::Tag;