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/0001-postgresql-over-sqlite.md

@@ -0,0 +1,40 @@
+# Postgres replaces SQLite as the primary datastore
+
+Status: accepted
+
+The project is moving from a single-reader tracker to a service published to a community,
+so we replaced `modernc.org/sqlite` with Postgres (`jackc/pgx/v5`, still pure Go, so
+`CGO_ENABLED=0` and the distroless image are unaffected). The deciding reason is future
+supportability — managed hosting, a datastore that survives the app outgrowing one box —
+**not** concurrency, which was measured and found to be a non-issue.
+
+## Considered options
+
+**Stay on SQLite.** Benchmarked against the real store at 1,500 rows (≈50 readers × 30
+series): ~9,700 upserts/sec single-writer, plateauing at ~780/sec under 8–50 concurrent
+writers, with `List()` holding 90–103 calls/sec under continuous write load. Projected
+real load at 50 readers is ~0.02 writes/sec — roughly four and a half orders of magnitude
+of headroom. `SetMaxOpenConns(1)` serialises writes but was shown not to starve reads;
+an apparent read collapse traced to row count and per-row scanning, not lock contention.
+SQLite would have worked. It was rejected for where the project is going, not for what
+it does today.
+
+**Postgres.** Chosen. Migrating is cheapest now — 29 rows in one table — and gets
+materially harder once there are live readers and a multi-tenant schema.
+
+## Consequences
+
+- Every statement in `internal/store` is rewritten: `?` → `$N`, `IS NOT` →
+  `IS DISTINCT FROM` (this one is load-bearing; it implements the `updated_at`
+  ordering rule), `pragma_table_info` → `information_schema.columns`,
+  `INTEGER`/`REAL` → `bigint`/`double precision`, `favorite` int-as-bool → `boolean`.
+- ~75 tests currently get a free isolated database from `t.TempDir()`. They now need a
+  live server, which makes Docker a hard prerequisite for `go test ./...`. This is the
+  permanent cost of the decision and the main reason it was close.
+- Backups get worse, not better: `VACUUM INTO` produced one self-contained file;
+  restoring now means `pg_dump`/`pg_restore`, a role, and a password.
+- A second stateful container joins the VPS alongside the existing headless-shell.
+- **Postgres does not address the real scaling limit.** At batch 14 per 10-minute tick
+  the poller checks at most 84 series/hour; 50 readers × 30 series is 1,500 bookmarks,
+  an 18-hour sweep against a configured 1-hour cooldown. That ceiling is an outbound
+  fetch budget and is fixed by deduplicating polls per Series, not by the datastore.