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

CONTEXT.md

@@ -0,0 +1,69 @@
+# Bookmark Manager
+
+Read-progress tracker for serialised fiction. A reader browses third-party manga and
+novel sites; userscripts capture where they got to and sync it to a self-hosted backend,
+so progress survives across sites and devices.
+
+## Language
+
+**Series**:
+One ongoing work — a manga or a novel — as published by a Site. Identified by its
+stable slug on that Site, never by its title. A Series exists once and is shared by
+every Reader who bookmarks it; it owns the facts that are true regardless of who is
+reading — title, cover, Latest Chapter. A Reader cannot change them; they describe the
+Series, not anyone's relationship to it.
+_Avoid_: manga, title, book, comic
+
+**Site**:
+One third-party source a Series is published on. A Series on two Sites is two Series.
+_Avoid_: source, host, provider, domain
+
+**Reader**:
+A person with their own Progress. Exactly one per set of credentials, so there is no
+separate "account" concept to model — the credential belongs to the Reader.
+_Avoid_: user, account, member, subscriber
+
+**Bookmark**:
+One Reader's tracked relationship with one Series, holding only what differs between
+Readers: Progress, Favourite, Lifecycle bucket. Facts about the Series itself belong
+to the Series, not here.
+_Avoid_: entry, item, record, subscription
+
+**Library**:
+One of the two halves of the collection — manga or novel — selected by a Bookmark's
+`kind`. The web UI and the userscripts each address exactly one Library at a time.
+Not a per-person concept: "everything one person has bookmarked" is a different idea
+and must not be called a Library.
+_Avoid_: section, tab, category
+
+**Progress**:
+The furthest chapter a reader has actually read in a Series. Only a change in Progress
+is real activity, so only Progress reorders the list.
+_Avoid_: position, bookmark (the noun is taken), last read
+
+**Latest Chapter**:
+The newest chapter a Site has published for a Series, discovered without the reader
+present. Distinct from Progress in every way that matters: it is a fact about the Site,
+not about the reader, and it must never reorder the list.
+_Avoid_: newest, current chapter, update
+
+**Poll**:
+The backend's own check of a Site for a Series's Latest Chapter, made without the
+Reader present. Performed once per Series no matter how many Readers bookmarked it —
+a Poll is work done on behalf of the Series, never on behalf of a Reader.
+_Avoid_: scrape, refresh, check, sync
+
+**New Chapter**:
+The state where Latest Chapter is ahead of Progress. The single condition the ember
+accent is permitted to signal.
+_Avoid_: unread, update available
+
+**Lifecycle bucket**:
+Which of three mutually exclusive states a Bookmark sits in — reading, archived, or
+finished. A Bookmark is in exactly one. Orthogonal to being a favourite.
+_Avoid_: state, status (as a domain word), list
+
+**Favourite**:
+A reader's manual pin on a Bookmark. Orthogonal to the Lifecycle bucket, and never a
+reason to reorder the list.
+_Avoid_: starred, pinned, priority