Add invoices, quotes, payments, products, and expenses

Five more resources following clients.rs's template exactly: the
same list/get/create/update/delete shape, including the
fetch-merge-write pattern for update. quotes.rs is otherwise
identical to invoices.rs since Quote is a type alias for Invoice in
the SDK (same table, distinguished by entity_type upstream).

Verified against a live local server: full create/list/get/update/
delete round trip for all six resources, confirming update preserves
fields the caller didn't mention.

Also adds CHANGELOG.md (Keep a Changelog format, matching the SDK's)
and updates README/CLAUDE.md's resource counts.

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

CLAUDE.md

@@ -20,8 +20,10 @@ design call, the same way its counterpart in the SDK works. If a change
 would contradict a stated goal or settle an open question, update
 `GOALS.md` in the same change.
 
-**Early scaffold.** Only the `clients` resource is wired up, as the template
-for the other ~34. See "Adding a resource" below before writing the next one.
+**Early scaffold.** Six resources are wired up (`clients`, `invoices`,
+`quotes`, `payments`, `products`, `expenses`) out of ~34; `clients` was the
+template the other five followed. See "Adding a resource" below before
+writing the next one.
 
 ## Development environment