Per-Reader userscript credential with UI install and rotation (#24) (#32)

Closes #24. Child of #18; based on current main (includes Postgres, Reader table, Discord OAuth).

## What

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` (new `token_epoch` column, migration 0006). One credential authenticates the script download path and the API bearer header.

- `internal/token`: derivation + hashing; the seed refreshes the owner's epoch-0 hash only before first rotation, so a restart can never resurrect a rotated-away credential
- `httpmw.Auth`/`ResolveReader`: acting Reader resolved from the credential hash, stashed in request context; the retired global `API_TOKEN` resolves to the owner until `API_TOKEN_GRACE_UNTIL` (enforced in code, 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 installed devices self-migrate on their next update poll
- Web UI: "Userscripts" panel — session-gated install endpoints render the script directly (credential never in markup, address bar, or a redirect), confirm-gated rotation with an atomic epoch bump + hash rewrite and a reinstall warning
- Both userscripts carry `__API_TOKEN__` placeholders; the committed global-token literal is removed

## Design note

Credentials are derived rather than stored-random because the server must rebuild install URLs after restarts while the DB holds only hashes. HMAC output is high-entropy and unbrute-forceable; the AC's intent (unguessable, DB-leak-proof) is met.

## Deploy (also in DEPLOY.md)

1. Add `TOKEN_KEY` (`openssl rand -hex 32`) — required; changing it later invalidates every credential.
2. Keep `API_TOKEN` + set `API_TOKEN_GRACE_UNTIL` for the 14-day window.
3. After deploy, sign in → Userscripts → reinstall both scripts on every device. This also retires the old global credential for real — its literal survives in git history (present since 0ef5286), so rotation is what kills it.

## Verification

- Full Go suite green against real Postgres per test; userscript JS suite 45/45
- New router-level tests: per-Reader isolation (read/write/delete), grace expiry on bearer + script path, self-migrating legacy path, install serving, rotation (old cred 401/404, new cred works, install renders new credential), app page leaks no credential
- Store tests: hash lookup, token info, atomic rotation with stale-epoch rejection, rotation survives restart
- Live smoke of the built binary: grace acceptance logged, derived auth, substitution, restart resilience, stored hash = SHA-256 of derived credential

Reviewed-on: #32
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

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