feat(backend): Discord OAuth login with DB-backed sessions (#23) (#31)

Implements #23 per ADR-0002.

- Discord authorization code grant (identify + guilds.members.read), form-encoded token exchange
- Guild membership gate via the single-guild endpoint; optional DISCORD_REQUIRED_ROLE (empty default)
- Owner Discord ID is the only identity allowed to sign in
- Sessions are DB rows with opaque random ids; cookie carries only the id; expiry enforced; delete = revoke
- HMAC session signing, derived key, and WEB_PASSWORD removed; no replacement signing secret
- Login rate limiting preserved on the callback
- Full flow tested through the real router against a local Discord stub (DISCORD_API_BASE)
- Env: DISCORD_CLIENT_ID/_CLIENT_SECRET/_GUILD_ID/_REQUIRED_ROLE/_API_BASE/_REDIRECT_URI; docs updated

go test ./... passes.

Reviewed-on: #31
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/AGENTS.md

@@ -38,14 +38,17 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   and enforces the ownership rule: client `title`/`series_url`/`cover` are
   written only when the series row is new (ADR-0003).
 - **Endpoints:** `GET /bookmarks`, `PUT /bookmarks/{key}` (upsert; see `updated_at` rule below), `DELETE /bookmarks/{key}`, `GET /healthz` (no auth).
-- **Web UI:** same binary serve password-gated browser UI on second
+- **Web UI:** same binary serve the browser UI on a second
   hostname — `GET /` (list, or login page when no session),
-  `POST /login`, `POST /logout`, `GET /static/*`, htmx fragment endpoints
+  `GET /auth/discord` + `GET /auth/discord/callback` (Discord OAuth,
+  ADR-0002), `POST /logout`, `GET /static/*`, htmx fragment endpoints
   under `/ui/*`. Templates + assets `go:embed`-ed under
   `backend/internal/web/`, so `backend/Dockerfile` must copy the whole
-  `internal/` tree, not just `*.go`. Sessions stateless
-  HMAC cookies keyed off `API_TOKEN`; `WEB_PASSWORD` gates them, and when empty,
-  web routes not registered at all. UI mutations read-modify-write
+  `internal/` tree, not just `*.go`. Sessions are rows in the `sessions`
+  table: the cookie carries only an opaque id, looked up (and expiry-
+  checked) on every request, and deleting the row revokes the session.
+  The owner's Discord ID is the only identity that can sign in while
+  registration is closed. UI mutations read-modify-write
   through `Store.Get` + `Store.Upsert` so `updated_at` rule stays one
   place. See `docs/superpowers/specs/2026-07-25-web-ui-design.md`.
   **Design-tool caveat:** templates link `/static/style.css` root-absolutely
@@ -101,8 +104,10 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
 - **Config via env:** `API_TOKEN`, `OWNER_DISCORD_ID` (seeds the owner Reader;
   required), `ALLOWED_ORIGINS` (comma list),
   `DATABASE_URL` (Postgres connection URL, required — no default),
-  `PORT` (default `8080`), `WEB_PASSWORD`
-  (gates browser UI; unset disable it),
+  `PORT` (default `8080`), `DISCORD_CLIENT_ID`/`_CLIENT_SECRET`/`_GUILD_ID`/
+  `_REDIRECT_URI` (required; Discord OAuth for the browser UI),
+  `DISCORD_REQUIRED_ROLE` (optional role gate, empty by default),
+  `DISCORD_API_BASE` (default `https://discord.com/api/v10`),
   `LATEST_CHAPTER_POLL_ENABLED`/`_COOLDOWN`/`_INTERVAL`/`_BATCH`/`_STAGGER`
   (background latest-chapter poller; defaults on, `1h`/`10m`/`14`/`20s`).
   `USERSCRIPT_PATH` and `NOVEL_USERSCRIPT_PATH` (files served at