[T4-6] Reader text-size preference

[liangricky7]

Blocked by: P0-1 (#1), P0-2 (#2) · Blocks: F-4 (#29)

Frontend ticket F-4 (#29) stores the reader's text size on their member record, not only in the browser, so v2's many-user case needs no migration. This is the endpoint it saves to.

• GET /v1/me returns the current member, including readerTextSize
• PUT /v1/me/preferences { readerTextSize } accepts small, medium or large. Anything else returns 400
• Stored on the members row, in the column P0-2 ([P0-2] The v1 schema, in one migration #2) creates
• Response shapes in packages/contracts

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Source: docs/backend-tickets.md