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>

.env.example

@@ -30,17 +30,28 @@ POSTGRES_PASSWORD=changeme-generate-a-long-random-password
 # TRAEFIK_ENTRYPOINT=websecure
 # TRAEFIK_CERTRESOLVER=le
 
-# --- Web UI ---
-# Password for the browser UI at https://$BOOKMARK_WEB_HOST. Leave unset to
-# disable the web UI entirely (the routes are not registered at all).
-# Generate one: openssl rand -base64 18
-WEB_PASSWORD=
+# --- Web UI (Discord OAuth) ---
+# Sign-in is a Discord authorization code grant (ADR-0002): the owner signs in
+# with Discord, and guild membership gates access. Create the application at
+# https://discord.com/developers/applications and register the exact callback
+# URL ($BOOKMARK_WEB_HOST/auth/discord/callback) as an OAuth2 redirect.
+DISCORD_CLIENT_ID=
+DISCORD_CLIENT_SECRET=
+# The guild whose membership gates sign-in (Developer Mode -> right-click the
+# server -> Copy Server ID).
+DISCORD_GUILD_ID=
+# Exact callback URL, e.g. https://bookmark.example.com/auth/discord/callback.
+# Discord matches it verbatim, so it must equal the registered redirect.
+DISCORD_REDIRECT_URI=
+# Optional: a role snowflake members must hold on top of guild membership.
+# Empty (the default) means membership alone suffices.
+# DISCORD_REQUIRED_ROLE=
 
-# Subdomain Traefik routes to the browser UI (required by the prod override,
-# whether or not WEB_PASSWORD is set). Left commented on purpose: an example
-# value here would be a silent wrong-hostname fallback, and Traefik would
-# publish the UI router on a domain you do not own. The same container also
-# answers on BOOKMARK_API_HOST for the userscript's API.
+# Subdomain Traefik routes to the browser UI (required by the prod override).
+# Left commented on purpose: an example value here would be a silent
+# wrong-hostname fallback, and Traefik would publish the UI router on a domain
+# you do not own. The same container also answers on BOOKMARK_API_HOST for the
+# userscript's API.
 # BOOKMARK_WEB_HOST=bookmark.example.com
 
 # --- Latest-chapter poller ---