feat(backend): give every Bookmark an owner (Reader table) (#22)

A readers table appears, keyed by Discord user ID and carrying the SHA-256
of the owner's userscript token (the global API token today). Startup seeds
exactly one Reader from OWNER_DISCORD_ID, idempotently, and a run-once
migration (0004, version-table-gated) attaches existing bookmarks to it
before reshaping: the surrogate key column is dropped and bookmarks are
keyed (reader_id, site, series_id) with an FK to readers ON DELETE CASCADE,
so a duplicate bookmark for one Reader and Series is impossible at the
database level.

Every store read and write is now scoped to the reader it names; handlers
act as the seeded owner while the global token remains the only credential.
Authentication and the wire format are untouched: the flat JSON still
carries key/site/series_id, with key derived on read.

OWNER_DISCORD_ID is a new required env var (compose + docs updated).

docker-compose.yml

@@ -15,6 +15,8 @@ services:
     environment:
       # API_TOKEN is required — compose refuses to start without it.
       API_TOKEN: ${API_TOKEN:?set API_TOKEN in .env}
+      # Owner's Discord user ID — required, seeds the one Reader row.
+      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.