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

The browser UI signs in with a Discord authorization code grant
(identify + guilds.members.read) instead of a shared password. Guild
membership is the gate; the owner's Discord ID is the only identity
allowed in while registration is closed. Sessions become rows in a
sessions table with opaque random ids — the cookie carries only the id,
looked up and expiry-checked per request — so deleting a row revokes a
session. HMAC cookie signing, its derived key, and WEB_PASSWORD are
gone, and no replacement signing secret is introduced (ADR-0002).

Discord's API base is configurable (DISCORD_API_BASE); the full flow is
tested through the real router against a local stub, including the
form-encoded token exchange Discord rejects if sent as JSON.

REDEPLOY.md

@@ -385,7 +385,7 @@ panel works on the phone.
 | UI looks like plain Georgia / system sans | `static/fonts/` missing from the image, or the browser cached an old `style.css`. `/static/*` is served `max-age=3600`, so hard-reload or wait an hour. |
 | CSS or template change did not appear | You restarted without `--build`. Assets are `//go:embed`ed. |
 | Font answers `application/octet-stream` | Old binary — the `.woff2` MIME registration is in `web.go`. Rebuild. |
-| Everyone logged out of the web UI | `API_TOKEN` or `WEB_PASSWORD` changed; sessions are derived from both. Expected, just log in again. |
+| Everyone logged out of the web UI | The `sessions` table was wiped; sessions are database rows, not signed cookies. Expected after a deliberate revoke. |
 | `compose` errors about `BOOKMARK_WEB_HOST` | Run from the directory holding `.env`. Both host vars are required even when the web UI is unused. |
 | Userscript did not update on the phone | Violentmonkey polls on its own schedule; force a check. `@version` comes from the file's mtime, so confirm the pull actually touched it. |
 | `bookmark-api` crash-loops, log says `password authentication failed for user "bookmarks"` | `POSTGRES_PASSWORD` in `.env` no longer matches the one burned into `postgres-data` at first init — Postgres reads that variable only when initialising an empty volume. Put the old value back, or reset the role: `$COMPOSE exec postgres psql -U bookmarks -d bookmarks -c '\password bookmarks'` (prompts, so nothing lands in shell history) and then match `.env` to it. |