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/templates/setup.html

@@ -0,0 +1,27 @@
+{{/* The userscript install panel. Each link serves the script rendered
+     with the acting Reader's credential inside it, so the credential never
+     appears in this page's markup, the address bar, or a redirect. Rotation
+     is confirm-gated because it invalidates every installed copy at once;
+     the response swaps this same panel open with the reinstall warning. */}}
+{{define "setup"}}
+<details class="setup" id="setup"{{if .Rotated}} open{{end}}>
+  <summary>Userscripts</summary>
+  <p class="setup-copy">Install each script once per device. They keep your
+    bookmarks in sync across every site and update themselves from here.</p>
+  <p class="setup-links">
+    <a class="ghost" href="/install/manga-bookmark.user.js">Install Manga script</a>
+    <a class="ghost" href="/install/novel-bookmark.user.js">Install Novels script</a>
+  </p>
+  {{if .Rotated}}
+  <p class="setup-warn" role="status">Credential rotated — the old one no
+    longer works. Reinstall both scripts on every device now, or they will
+    silently stop syncing.</p>
+  {{else}}
+  <form class="setup-rotate" hx-post="/rotate-token" hx-target="#setup"
+        hx-swap="outerHTML"
+        hx-confirm="Rotation invalidates the current credential on every device immediately. You will have to reinstall both scripts everywhere. Rotate?">
+    <button type="submit" class="ghost">Rotate credential</button>
+  </form>
+  {{end}}
+</details>
+{{end}}