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/web/static/style.css

@@ -243,6 +243,53 @@ button { cursor: pointer; }
 /* The label is 15px tall by design; the thumb gets 44 without moving it. */
 .ghost::after { content: ""; position: absolute; inset: -15px -12px; }
 
+/* ---- userscript setup: collapsed by default, one hairline, no card ---- */
+.setup {
+  margin: 0 20px;
+  padding: 12px 0 0;
+  border-bottom: 1px solid var(--rule);
+  color: var(--mute);
+}
+.setup summary {
+  display: flex;
+  align-items: center;
+  min-height: 44px;
+  padding: 0;
+  font: 500 10px/1 var(--font-mono);
+  letter-spacing: .2em;
+  text-transform: uppercase;
+  color: var(--mute-2);
+  cursor: pointer;
+  list-style: none;
+}
+.setup summary::-webkit-details-marker { display: none; }
+.setup summary:hover { color: var(--paper); }
+.setup[open] { padding-bottom: 16px; }
+.setup-copy {
+  margin: 0;
+  padding: 4px 0 12px;
+  font: 14px/1.55 var(--font-body);
+  color: var(--mute);
+}
+.setup-links {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px 20px;
+  margin: 0 0 14px;
+}
+.setup-links .ghost { font-size: 11px; }
+.setup-rotate { margin: 0; }
+/* Rotation confirmation: the one hot state the panel wears, and it is
+   destruction, not new-chapter signal — danger, never ember. */
+.setup-warn {
+  margin: 0;
+  padding: 10px 12px;
+  border: 1px solid var(--danger);
+  color: var(--danger);
+  font: 500 12px/1.5 var(--font-mono);
+  letter-spacing: .04em;
+}
+
 .chrome { display: flex; flex-direction: column; }
 
 .searchbar {