feat(backend)!: run on Postgres with a migration-owned schema (#28)

Swap modernc.org/sqlite for jackc/pgx/v5 with no observable change:
same endpoints, same wire format, same updated_at ordering rule.

The schema now comes from numbered SQL embedded in the binary and
applied on startup, one transaction each, recorded in
schema_migrations. That replaces two pieces of SQLite-era machinery,
both deleted rather than ported: the column probing (Postgres has ADD
COLUMN IF NOT EXISTS, and there is no legacy database left to probe)
and the Asura key rewrite, which has run clean on every start for
months now that the userscripts strip build hashes before writing. Its
regexp survives as latest.asuraBuildHash, where the poller still needs
it to scope chapter links to a series whose slug carries a rotating
hash.

Types get real: favorite is a boolean, chapter numbers double
precision, timestamps stay unix-ms bigint. SQLite's null-safe IS NOT
becomes IS DISTINCT FROM, which is what implements the rule that only
reading progress reorders a list. Inside COALESCE/NULLIF the status
and kind parameters need an explicit ::text -- there is no target
column to infer from and Postgres refuses to guess.

Tests lose their free t.TempDir() database, so Docker is now a hard
prerequisite for `go test ./...`: internal/pgtest starts one
postgres:17-alpine per test binary and hands each test a database of
its own.

Also lands CONTEXT.md and the four ADRs written while scoping #18.

BREAKING CHANGE: DB_PATH is retired for DATABASE_URL, which is
required and has no default. Compose gains a postgres service on an
internal network with its own volume; POSTGRES_PASSWORD joins .env.
The old bookmarks-data volume is deliberately left undeclared so
`docker compose down -v` cannot take the pre-migration database with
it. main is not deployable until #25 and #26 land.

Closes #20

Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

AGENTS.md

@@ -25,7 +25,7 @@ Userscript targets **Violentmonkey**, so `GM_*` APIs available, but stay GM-free
 
 ```
 Two Violentmonkey userscripts (isolated world, per-site adapters, localStorage cache)
-   -- fetch() HTTPS -->  reverse proxy (TLS + CORS)  -->  Go net/http  -->  SQLite (volume)
+   -- fetch() HTTPS -->  reverse proxy (TLS + CORS)  -->  Go net/http  -->  Postgres (volume)
 ```
 
 Backend-specific architecture (packages, endpoints, poller, config env vars) lives in `backend/AGENTS.md`. Userscript-specific structure (adapters, retry queue, UI, live URL shapes) lives in `userscript/AGENTS.md`.
@@ -33,11 +33,11 @@ Backend-specific architecture (packages, endpoints, poller, config env vars) liv
 ## Commands
 
 Backend (`cd backend`):
-- Test all: `go test ./...`
+- Test all: `go test ./...` — **needs Docker.** Each test package starts a throwaway `postgres:17-alpine` container (`internal/pgtest`).
 - Single test: `go test -run TestName ./...`
 - Build static binary: `CGO_ENABLED=0 go build`
 
-Local stack: `docker compose up` (named volume mounted at `/data`, `restart: unless-stopped`).
+Local stack: `docker compose up` (bookmark-api + postgres + headless-shell; `postgres-data` named volume, `restart: unless-stopped`).
 
 Smoke test: `curl` endpoints with `Authorization: Bearer <token>`; confirm `OPTIONS` preflight return CORS headers and `/healthz` return 200.
 
@@ -79,7 +79,7 @@ Anchored to OWASP Top 10 / ASVS. Every rule below already has a working example
 
 Go backend:
 
-- SQL always parameterized (`?`). Only compile-time constants (`bookmarkColumns`) may be concatenated into query text — never a request value, not even a validated one.
+- SQL always parameterized (`$N`). Only compile-time constants (`bookmarkColumns`) may be concatenated into query text — never a request value, not even a validated one.
 - `html/template` only for anything a browser parses, never `text/template`. Never wrap stored or fetched strings in `template.HTML`/`JS`/`URL`; that switches off the escaping every template depends on.
 - Any outbound fetch of a client-supplied URL passes `fetchableSeriesURL` (site + `https` + host check) first. `series_url` arrives in a PUT body, so without the gate the poller will probe arbitrary hosts from the server's own network position. New fetch path reuses the gate rather than re-deriving one.
 - Cap every remote body with `io.LimitReader` (`maxBodyBytes`). An unbounded read is an OOM handed to whatever is on the other end.
@@ -133,6 +133,22 @@ Test: "competent reader get this from code in few sec?" Yes → skip. Needs deto
 
 `golang-code-style`, `golang-error-handling`, `golang-performance`, `golang-testing` for backend Go work.
 
+## Agent skills
+
+`AGENTS.md` is the single source of truth for agent guidance; every `CLAUDE.md` in this repo is a symlink to the `AGENTS.md` beside it. Edit `AGENTS.md`.
+
+### Issue tracker
+
+Issues live as Gitea issues on `gitea.violetcrown.my.id` (`sulthan/mangaBookmark`), driven by the `tea` CLI — not `gh`. See `docs/agents/issue-tracker.md`.
+
+### Triage labels
+
+Default five-role vocabulary, label strings unchanged (`needs-triage`, `needs-info`, `ready-for-agent`, `ready-for-human`, `wontfix`). See `docs/agents/triage-labels.md`.
+
+### Domain docs
+
+Single-context: one root `CONTEXT.md` plus `docs/adr/`, both created lazily. See `docs/agents/domain.md`.
+
 ## graphify
 
 Project has knowledge graph at graphify-out/ with god nodes, community structure, cross-file relationships.