Give every Bookmark an owner (Reader table) (#30)

Closes #22

## What

A `readers` table appears; every Bookmark belongs to one. The owner is seeded as the first and only Reader, and all existing rows are attached to them.

- **Migration 0003**: `readers` (discord_id UNIQUE, token_sha256 UNIQUE, created_at).
- **Migration 0004** (run-once, version-table-gated): attaches existing bookmarks to the seeded owner, drops the surrogate `key` column, composite PK `(reader_id, site, series_id)`, FK to readers `ON DELETE CASCADE` — a duplicate Bookmark for one Reader and Series is impossible at the database level.
- **Seed**: `Store.Open` runs schema to 0003, seeds exactly one owner row from `OWNER_DISCORD_ID` (hash = SHA-256 of `API_TOKEN`, refreshed on every start so rotation stays current), then migrates the rest.
- **Scoping**: `List/Get/Upsert/Delete` take `readerID`; the wire `key` is derived as `site:series_id` on read. Handlers act as `Store.OwnerID()` while the global token remains the only credential.
- **Unchanged**: authentication and the flat wire format — nothing observable changes from outside.
- **New env** `OWNER_DISCORD_ID` (required): compose, .env.example, DEPLOY.md, README.md, backend/AGENTS.md updated.

Series-level methods (due queue, mark-checked, set-latest-chapter) stay unscoped deliberately: series are shared rows polled once per due cycle, and the reader_count ordering requires cross-reader visibility (ADR-0003).

## Verification

- `go test ./...` green, including new tests: seed idempotency + hash refresh, 0004 attach migration, DB-level duplicate impossibility, per-reader scoping, reader-delete cascade.
- Live smoke test on fresh Postgres: seed → PUT/GET (flat wire intact) → restart idempotent; stored hash matches SHA-256 of the token.

## Deploy note

`OWNER_DISCORD_ID` is required after this lands — the backend refuses to start without it. Set it to the owner's Discord snowflake (Settings → Advanced → Developer Mode → right-click name → Copy User ID).

Reviewed-on: #30
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/web/web.go

@@ -32,6 +32,9 @@ const RecentCount = 5
 // representations (HTML versus JSON) to different clients under different auth.
 type Handler struct {
 	store    *store.Store
+	// readerID is the Reader this UI acts as — the seeded owner, while the web
+	// password is still the only credential (issue #22).
+	readerID int64
 	tmpl     *template.Template
 	key      []byte
 	password string
@@ -81,13 +84,14 @@ type loginView struct {
 
 // New parses every template up front so a broken one kills the process at
 // startup rather than the first request that touches it.
-func New(s *store.Store, apiToken, webPassword string) (*Handler, error) {
+func New(s *store.Store, readerID int64, apiToken, webPassword string) (*Handler, error) {
 	tmpl, err := template.ParseFS(templateFS, "templates/*.html")
 	if err != nil {
 		return nil, err
 	}
 	return &Handler{
 		store:    s,
+		readerID: readerID,
 		tmpl:     tmpl,
 		key:      session.Key(apiToken, webPassword),
 		password: webPassword,
@@ -216,7 +220,7 @@ func libOf(q string) string {
 // archived favourite therefore shows only under Archived: Favourites means
 // "favourites I am currently reading".
 func (h *Handler) buildListView(lib, tab string) (listView, error) {
-	all, err := h.store.List() // already ordered updated_at DESC
+	all, err := h.store.List(h.readerID) // already ordered updated_at DESC
 	if err != nil {
 		return listView{}, err
 	}
@@ -374,7 +378,7 @@ func (h *Handler) loadForMutation(w http.ResponseWriter, r *http.Request) (store
 		http.Error(w, "missing key", http.StatusBadRequest)
 		return store.Bookmark{}, false
 	}
-	b, ok, err := h.store.Get(key)
+	b, ok, err := h.store.Get(h.readerID, key)
 	if err != nil {
 		log.Printf("ui get %q: %v", key, err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -397,7 +401,7 @@ func (h *Handler) loadForMutation(w http.ResponseWriter, r *http.Request) (store
 // describe the whole library, so they are rebuilt out of band on every
 // mutation, at the cost of one extra list read per toggle.
 func (h *Handler) saveAndRenderCard(w http.ResponseWriter, r *http.Request, b store.Bookmark) {
-	stored, err := h.store.Upsert(b)
+	stored, err := h.store.Upsert(h.readerID, b)
 	if err != nil {
 		log.Printf("ui upsert %q: %v", b.Key, err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -489,7 +493,7 @@ func (h *Handler) uiDelete(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "missing key", http.StatusBadRequest)
 		return
 	}
-	if err := h.store.Delete(key); err != nil {
+	if err := h.store.Delete(h.readerID, key); err != nil {
 		log.Printf("ui delete %q: %v", key, err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
 		return