Open registration to guild members (#27) (#36)

Closes #27.

Guild membership is now the whole gate. `discordCallback` checks membership
(and `DISCORD_REQUIRED_ROLE` when set), then `Store.EnsureReader` creates the
Reader on first sight and returns the same row on every later login. The
refusal returns before `EnsureReader`, so a turned-away sign-in leaves no row
behind. `OWNER_DISCORD_ID` still seeds the owner, but only as the
administrator — it no longer gates login.

The cutover grace path goes with it: `API_TOKEN`, `API_TOKEN_GRACE_UNTIL` and
the legacy branch in `httpmw.ResolveReader` are deleted, so a credential
authenticates exactly one Reader or nothing. `userscript.Handler` drops its
re-derivation too — the resolved path segment is already the credential.

New surfaces: an empty library offers both install links (behind the
tab-specific empty states, so "No favourites yet" still wins), and the owner
alone gets a Readers panel with `POST /readers/{id}/revoke`. The owner's own
row is not revocable — 404, not a self-logout.

Isolation is asserted from both directions for read, modify and delete, and
the shared-series invariant is pinned: two Readers on one series produce one
series row, two independent progresses, one poll per due cycle, and one
Reader's delete leaves the other's bookmark and the poll intact.

Verified: `go test ./...` green; live smoke against a throwaway Postgres —
empty-library state in both colour branches, roster rendering, a real revoke
through the panel (target 401s next request, owner untouched), owner
self-revoke refused 404, per-Reader `/u/<cred>` and bearer auth both 200 with
404 for an unknown credential.

Reviewed-on: #36
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

docker-compose.yml

@@ -16,19 +16,17 @@ services:
       # 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's Discord user ID — required. Seeds the owner Reader (the
+      # administrator); every other Reader registers on their first login.
       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}
       # The bookmarks database. Host is the compose service name; the password
       # comes from .env so it is never committed.
       DATABASE_URL: ${DATABASE_URL:-postgres://bookmarks:${POSTGRES_PASSWORD:?set POSTGRES_PASSWORD in .env}@postgres:5432/bookmarks?sslmode=disable}
       PORT: "8080"
-      # Discord OAuth for the browser UI (issue #23). The first four are
+      # Discord OAuth for the browser UI (ADR-0002). The first four are
       # required; DISCORD_REQUIRED_ROLE is optional and empty by default.
+      # Guild membership is the whole gate: any member becomes a Reader.
       DISCORD_CLIENT_ID: ${DISCORD_CLIENT_ID:?set DISCORD_CLIENT_ID in .env}
       DISCORD_CLIENT_SECRET: ${DISCORD_CLIENT_SECRET:?set DISCORD_CLIENT_SECRET in .env}
       DISCORD_GUILD_ID: ${DISCORD_GUILD_ID:?set DISCORD_GUILD_ID in .env}