v2 P4: Typed response models

Part of the v2 roadmap (pillar P4 — largest surface area).

Extends the discipline the endpoint classes already apply to GET responses (typed properties over raw dicts) to everywhere a dict[str, Any] still leaks through — request payload construction, feed items, receipt items, webhook bodies.

Scope

• Touches every endpoint's constructor and payload-building code.
• Recommend stdlib dataclasses over adding a validation library (pydantic/attrs), to keep httpx as the only new dependency — worth confirming before implementation starts.

Flag: #90 (exposing Monzo's raw account_type string instead of the current incomplete enum) sits squarely inside this pillar's territory but has not been confirmed as in-scope. Needs an explicit yes/no before this starts, since typed-model work on Account will touch that field regardless.

Breaking: yes
Effort: L
Phase: beta

Start by reviewing the endpoint constructors and payload-building code described in the issue, then confirm whether stdlib dataclasses are appropriate and whether #90 is in scope. The work is done when the listed request payloads, feed and receipt items, webhook bodies, and remaining dict[str, Any] leaks use typed models, with the breaking changes accounted for.