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/api/handlers.go

@@ -13,6 +13,9 @@ import (
 // Handler serves the userscript-facing JSON bookmark API.
 type Handler struct {
 	Store *store.Store
+	// ReaderID is the Reader this request acts as. Authentication is still the
+	// single global token, so that is always the seeded owner (issue #22).
+	ReaderID int64
 }
 
 func writeJSON(w http.ResponseWriter, status int, v any) {
@@ -25,9 +28,9 @@ func writeJSON(w http.ResponseWriter, status int, v any) {
 	}
 }
 
-// List returns all bookmarks. GET /bookmarks
+// List returns all bookmarks of the acting Reader. GET /bookmarks
 func (h *Handler) List(w http.ResponseWriter, r *http.Request) {
-	items, err := h.Store.List()
+	items, err := h.Store.List(h.ReaderID)
 	if err != nil {
 		log.Printf("list: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -91,7 +94,7 @@ func (h *Handler) Put(w http.ResponseWriter, r *http.Request) {
 	// reading progress actually moved. Any client value is ignored.
 	b.UpdatedAt = time.Now().UnixMilli()
 
-	stored, err := h.Store.Upsert(b)
+	stored, err := h.Store.Upsert(h.ReaderID, b)
 	if err != nil {
 		log.Printf("upsert: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -109,7 +112,7 @@ func (h *Handler) Delete(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("delete: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
 		return