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>

AGENTS.md

@@ -70,7 +70,7 @@ instantly.
 
 Existing guarantees — don't regress:
 
-- Auth on `/bookmarks*`: require `Authorization: Bearer <API_TOKEN>`, **constant-time compare**, 401 otherwise.
+- Auth on `/bookmarks*`: require `Authorization: Bearer <credential>` — the acting Reader's credential, matched by SHA-256 against `readers.token_sha256` — **constant-time compare** (via the hash, never the secret itself), 401 otherwise.
 - CORS: reflect `Origin` only when in `ALLOWED_ORIGINS`; allow `GET,PUT,DELETE,OPTIONS` + headers `Authorization,Content-Type`; answer preflight `OPTIONS` with `204`.
 
 ## Secure coding rules (code you write here)
@@ -83,7 +83,7 @@ Go backend:
 - `html/template` only for anything a browser parses, never `text/template`. Never wrap stored or fetched strings in `template.HTML`/`JS`/`URL`; that switches off the escaping every template depends on.
 - Any outbound fetch of a client-supplied URL passes `fetchableSeriesURL` (site + `https` + host check) first. `series_url` arrives in a PUT body, so without the gate the poller will probe arbitrary hosts from the server's own network position. New fetch path reuses the gate rather than re-deriving one.
 - Cap every remote body with `io.LimitReader` (`maxBodyBytes`). An unbounded read is an OOM handed to whatever is on the other end.
-- Compare secrets with `hmac.Equal` / `subtle.ConstantTimeCompare`, never `==`. Covers the API token.
+- Compare secrets with `hmac.Equal` / `subtle.ConstantTimeCompare`, never `==`. Covers the retired global token during its grace window.
 - Errors: generic text to the client (`http.Error(w, "internal error", 500)`), detail to `log.Printf`. Never log `API_TOKEN`, `DISCORD_CLIENT_SECRET`, a session id, or a whole `Authorization` header.
 - Proxy headers are trusted only where they already are: `X-Forwarded-Proto` for the Secure cookie flag, **rightmost** `X-Forwarded-For` for client IP (leftmost is attacker-supplied). Don't read either anywhere else.
 - Session cookies keep `HttpOnly`, `SameSite`, `Secure`-when-HTTPS; expiry is enforced by the `sessions` table lookup, not a signature.
@@ -93,8 +93,8 @@ Go backend:
 Userscript:
 
 - Site-derived and stored strings render via `el(..., {text})` / `textContent`. `{html}` and `innerHTML` are for author-written literal markup only (`TEMPLATE`, `CSS`) — never a title, chapter label, or API response field. The page DOM belongs to a third-party site; treat it as attacker-controlled.
-- Isolated world protects the token from the site's JS. It does not protect anything from an `innerHTML` sink you add yourself.
-- The `API_TOKEN` literal sits in both userscripts and must equal backend `API_TOKEN`. Never copy it into logs, docs, commit messages, issues, or a new file. Rotation touches three places: backend env plus both scripts.
+- Isolated world protects the credential from the site's JS. It does not protect anything from an `innerHTML` sink you add yourself.
+- The userscripts carry `__API_TOKEN__` placeholders, substituted at serve time with the requesting Reader's credential (`internal/userscript`). Never put a real credential in the repo, docs, commit messages, or issues. Rotation is a web-UI action (epoch bump, `internal/token`); `TOKEN_KEY` in backend env is what derives every credential — never log it.
 - `fetch()` targets `API_BASE` only — no dynamic origin, no site-supplied URL. `authHeaders()` goes nowhere but the backend.
 - `localStorage` is shared with the site's own JS: cache and queue live there, credentials never do.
 - Wrap every `localStorage` read/write and `JSON.parse` in try/catch (quota, private mode, corrupt entry), as the existing helpers do.