feat(backend): per-Reader userscript credential with UI install and rotation (#24)

Each Reader's userscript credential is derived from TOKEN_KEY, their
Discord id and a token epoch (HMAC-SHA256, hex); only its SHA-256 sits in
readers.token_sha256, so install URLs survive restarts while a database
leak yields nothing but hashes. One credential authenticates the script
download path and the API bearer header.

- internal/token: derivation + hashing; migration 0006 adds token_epoch
- seed refreshes the owner's epoch-0 hash only before first rotation
- httpmw.Auth resolves the acting Reader from the credential hash and
  stashes it in the request context; the retired API_TOKEN resolves to
  the owner until API_TOKEN_GRACE_UNTIL, logged per use, on both the
  bearer and script-download paths
- userscript handler renders the bindmounted file with the resolved
  Reader's credential substituted for __API_TOKEN__; a legacy-path
  request during grace serves the derived credential, so devices
  self-migrate on their next update poll
- web UI: Userscripts panel with session-gated install endpoints that
  render the script directly (credential never in markup, address bar
  or a redirect) and confirm-gated rotation; atomic epoch bump + hash
  rewrite in the store
- both userscripts carry __API_TOKEN__ placeholders; the committed
  global-token literal is removed (rotating at deploy retires it for
  real — it survives in git history)
- env: TOKEN_KEY required, API_TOKEN/API_TOKEN_GRACE_UNTIL retire the
  legacy credential; docs and compose updated

backend/internal/api/handlers.go

@@ -7,15 +7,13 @@ import (
 	"strings"
 	"time"
 
+	"bookmarkmanager/backend/internal/httpmw"
 	"bookmarkmanager/backend/internal/store"
 )
 
 // 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) {
@@ -30,7 +28,7 @@ func writeJSON(w http.ResponseWriter, status int, v any) {
 
 // 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(h.ReaderID)
+	items, err := h.Store.List(httpmw.ReaderID(r))
 	if err != nil {
 		log.Printf("list: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -94,7 +92,7 @@ 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(h.ReaderID, b)
+	stored, err := h.Store.Upsert(httpmw.ReaderID(r), b)
 	if err != nil {
 		log.Printf("upsert: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
@@ -112,7 +110,7 @@ func (h *Handler) Delete(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "missing key", http.StatusBadRequest)
 		return
 	}
-	if err := h.Store.Delete(h.ReaderID, key); err != nil {
+	if err := h.Store.Delete(httpmw.ReaderID(r), key); err != nil {
 		log.Printf("delete: %v", err)
 		http.Error(w, "internal error", http.StatusInternalServerError)
 		return