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>

docker-compose.yml

@@ -13,8 +13,13 @@ services:
     container_name: bookmark-api
     restart: unless-stopped
     environment:
-      # API_TOKEN is required — compose refuses to start without it.
-      API_TOKEN: ${API_TOKEN:?set API_TOKEN in .env}
+      # TOKEN_KEY derives every Reader's userscript credential (issue #24) —
+      # compose refuses to start without it.
+      TOKEN_KEY: ${TOKEN_KEY:?set TOKEN_KEY in .env}
+      # Retired global credential, optional: only used until the grace
+      # deadline, for already-installed scripts. Remove after the window.
+      API_TOKEN: ${API_TOKEN:-}
+      API_TOKEN_GRACE_UNTIL: ${API_TOKEN_GRACE_UNTIL:-}
       # Owner's Discord user ID — required, seeds the one Reader row.
       OWNER_DISCORD_ID: ${OWNER_DISCORD_ID:?set OWNER_DISCORD_ID in .env}
       ALLOWED_ORIGINS: ${ALLOWED_ORIGINS:-https://asuracomic.net,https://asurascans.com,https://demonicscans.org,https://comix.to,https://kagane.to,https://novelfull.com,https://lightnovelworld.net}