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>

userscript/manga-bookmark.user.js

@@ -4,8 +4,8 @@
 // @version      1.6.0
 // @description  Track read progress on Asura, Demonic, Comix & Kagane and sync to a self-hosted backend. Bromite-compatible (no GM_* APIs).
 // @author       you
-// @downloadURL  https://bookmark-api.violetcrown.my.id/u/40d79969b5442f90df4fe306a092c7c50e7b4a7a98099f98cc398f4fb374b1df/manga-bookmark.user.js
-// @updateURL    https://bookmark-api.violetcrown.my.id/u/40d79969b5442f90df4fe306a092c7c50e7b4a7a98099f98cc398f4fb374b1df/manga-bookmark.user.js
+// @downloadURL  https://bookmark-api.violetcrown.my.id/u/__API_TOKEN__/manga-bookmark.user.js
+// @updateURL    https://bookmark-api.violetcrown.my.id/u/__API_TOKEN__/manga-bookmark.user.js
 // @match        https://asuracomic.net/*
 // @match        https://asurascans.com/*
 // @match        https://demonicscans.org/*
@@ -22,7 +22,7 @@
   // CONFIG — fill these in before installing.
   // ============================================================
   const API_BASE = "https://bookmark-api.violetcrown.my.id"; // your backend origin, no trailing slash
-  const API_TOKEN = "40d79969b5442f90df4fe306a092c7c50e7b4a7a98099f98cc398f4fb374b1df"; // must equal backend API_TOKEN
+  const API_TOKEN = "__API_TOKEN__"; // substituted by the backend at serve time (issue #24)
   const WEB_BASE = "https://bookmark.violetcrown.my.id"; // the browser UI, for the panel's nav chips
 
   // This script owns the manga library; the novel script is a separate install