Batch H1 steps 1-2 of the hardening plan. Wire contracts frozen; the
pinned shapes are unchanged modulo the whitelisted additive GET keys
(docs/contract_pins.md).
1. One canonical request struct. The web PUT binds SaveProgressRequest
directly (identity/attribution fields are json:"-" so they stay
server-owned) and copies its server fields once — the hand-rolled DTO
and the 20-line field-copy block are gone. Parse set for existing
clients is IDENTICAL to the old DTO (verified in web/src: the ebook
reader sends percentage/current_page/total_pages/reading_mode/epubcfi/
context_text/chapter/zoom_level — reader.ts:3113-3143; the audiobook
reader sends percentage/chapter/audio_offset_seconds/elapsed_seconds/
reading_mode — audio.ts:518-520).
2. GET /api/media-items/:id/progress now serializes through one
row-struct-driven builder instead of a hand-map, so a row column
cannot be silently forgotten again (the audio_offset_seconds
omission). Existing keys are verbatim; the row already carried
audio_offset_seconds/reading_mode/viewport_x-y/zoom_level/
scroll_position_x-y/panel_number and they surface as WHITELISTED
additive keys. The web audiobook reader already reads
audio_offset_seconds (audio.ts:219) — with the key absent,
durationless books resumed at chapter start (offset always 0); they
now resume correctly. The conditional context_text bolt-on and the
server-derived css_selector/anchor_href block are unchanged —
restore precedence stays percentage-first, never re-derived.