docs: record the domain model and the Postgres/OAuth ADRs

Written while scoping #18. CONTEXT.md pins the ubiquitous language
(Series, Reader, Bookmark, Progress, Latest Chapter, Poll) that the
schema split and the ordering rule are argued in; the four ADRs record
the decisions that follow from it, starting with Postgres over SQLite.

Refs #18

docs/adr/0004-wire-format-does-not-mirror-the-schema.md

@@ -0,0 +1,32 @@
+# The wire format stays flat and deliberately does not mirror the schema
+
+Status: accepted
+
+Storage splits a tracked series across two tables (ADR-0003), but `GET /bookmarks` and
+`PUT /bookmarks/{key}` keep emitting and accepting one **flat** JSON object with `title`,
+`cover`, `last_chapter` and `latest_chapter` as siblings — exactly the shape they had
+when there was one table. The server joins on the way out and decomposes on the way in.
+
+## Why a future reader will find this surprising
+
+The obvious move after splitting a table is to nest the JSON to match. Don't "fix" this.
+
+**A nested payload would have broken every installed userscript instantly.** Scripts read
+`b.title` directly; moving it to `b.series.title` yields `undefined` — no error, just
+blank rows and a New Chapter signal that silently reports nothing forever. Because
+Violentmonkey updates roughly once a day per device, the migration relies on old scripts
+continuing to work during a 14-day grace window. A nested format and that grace window
+are mutually exclusive.
+
+**It is also the better contract independently of compatibility.** A client rendering one
+row needs the title and the reading position together; nesting exports the re-stitching
+to every browser to mirror a decision about disk layout it should not know about. Keeping
+them separate lets storage change again later without a client release — which is the
+whole reason this ADR is worth the paragraph.
+
+## Consequence
+
+The flat shape is a contract, not an implementation detail. Changing the storage schema
+must not change it. It follows the rule already in force for `updated_at`: the server
+owns the truth and returns the row **as stored**, and clients adopt the response rather
+than their own payload.