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>

userscript/CLAUDE.md

@@ -1,64 +0,0 @@
-Guidance for Claude Code working under `userscript/`. See root `CLAUDE.md` for the project-wide architecture diagram, hard constraints, and design system.
-
-### Userscript structure (single IIFE, `manga-bookmark.user.js`)
-
-1. **Site adapters** — one per host, `detect(location, document)` return page `type` + IDs. Identify type/IDs from **URL regex** (most stable); pull `title`/`cover` from **`og:title`/`og:image` meta tags**, not CSS classes.
-2. **API client** — `apiGet/apiPut/apiDelete` with bearer header; `localStorage` key `bmgr:manga:cache` for instant render + offline fallback.
-3. **Progress logic** — auto-upsert `last_chapter` only when `chapterNum >= stored last_chapter_num` (re-reading old chapters must not regress progress; unparseable -> set current). Manual panel override forces any value.
-4. **Retry queue** — every write go through `pushBookmark`/`pushDelete`, so
-   failed mutation park in `localStorage` (`bmgr:manga:queue`) and replayed on
-   next navigation, reconnect, or `refresh()`. Entries are markers
-   (`{key, op, sendStatus, attempts}`), never payloads — body read from
-   cache at send time, so one entry per key give ordering and coalescing for
-   free. `sendStatus` is **sticky**: while archive pending, later writes to
-   that key keep carrying bucket, which stop successful
-   in-between write from silently un-archiving series. `refresh()` drains
-   before it fetches and overlays anything still pending, so list never
-   flaps. 400 drops entry, 401 abort pass and keep queue, and
-   transient failures retry to cap of 10. Latest-chapter writes deliberately
-   stay out of queue. See
-   `docs/superpowers/specs/2026-07-27-offline-retry-queue-design.md`.
-5. **UI** — rendered inside **Shadow DOM** root to isolate from site CSS
-   (critical on mobile). Three tabs (All / Favourites / Archived) and row of
-   link chips to web UI and both manga sites; `WEB_BASE` sits in CONFIG
-   block next to `API_BASE`. FAB is `7 × 44` edge tab whose *hit* area
-   widened to `28 × 72` by invisible `#hit` child; `#fab` must keep
-   `touch-action: none` and must **not** regain `overflow: hidden`. Since
-   `touch-action` resolved at gesture start, strip can't be both
-   browser-scrolled and script-dragged, so `makeDraggable` splits by intent: swipe
-   from `#hit` scrolls via `window.scrollBy`, hold of `ARM_MS` arms
-   reposition drag, visible sliver drags with no hold. See
-   `docs/superpowers/specs/2026-07-28-edge-tab-hitbox-design.md`.
-6. **SPA navigation** — Asura is Astro, client-routed on comic/chapter pages: patch `history.pushState`/`replaceState` + listen `popstate`, re-run `detect()` on URL change so auto-update fire without reload. Demonic uses classic reloads (initial `document-idle` run suffice).
-
-### Live URL shapes (verified 2026-07-26, may drift — re-check against live pages before trust)
-
-- **asurascans.com**: series `/comics/<slug>` (slug carries trailing
-  site-wide build-hash suffix, e.g. `-059befe1`, that **rotates on every
-  redeploy**), chapter `/comics/<slug>/chapter/<n>`. `seriesId` must strip
-  hash (`/-[0-9a-f]{8}$/`, `stripBuildHash` in userscript,
-  `asuraBuildHash` in backend); URLs keep full slug — stale-hash
-  URLs 302 to current ones. Astro-rendered; chapter links present in raw
-  server HTML.
-- **demonicscans.org**: series `/manga/<slug>` (slug may URL-encode punctuation, e.g. `%2527` for `'`), chapter `/title/<slug>/chapter/<n>/<page>` (older `chaptered.php?manga=<id>&chapter=<n>` form still exists as redirect, what series-page chapter-list anchors link through).
-  Encodings (incl. triple-encoded punctuation like `%25252D`) identical
-  on /manga/ and /title/ pages, so decode-once seriesIds match — verified
-  2026-07-28.
-- **novelfull.com** (novel script): series `/<slug>.html`, chapter
-  `/<slug>/chapter-<n>[-<title-slug>].html`. No `og:*` tags at all — title from
-  `h3.title` (series) or `a.truyen-title` (chapter), cover from
-  `meta[name="image"]`. Behind a Cloudflare JS challenge no TLS fingerprint
-  clears, so the backend polls it through the headless browser.
-- **lightnovelworld.net** (novel script): series `/novel/<slug>/`, chapter
-  `/<slug>-chapter-<n>/` — flat, at the site root. `h1.entry-title` is the clean
-  title on a series page and `<Title> Chapter <n>` on a chapter page. Chapter
-  pages carry no `og:image`. Its series page lists every chapter with an
-  absolute href, so the backend polls it with the plain TLS client.
-
-### Second script: `novel-bookmark.user.js`
-
-A copy of the manga script with two adapters, `LIBRARY = "novel"` and
-`STORE_PREFIX = "bmgr:novel:"`. No migration loop (this script has no previous
-installation to carry keys over from). Installed alongside the manga script;
-both write to the same backend with the same `LIBRARY` column discriminating
-them.

userscript/CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md