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).

DEPLOY.md

@@ -35,6 +35,11 @@ Edit `.env`:
 # Required — long random secret, also goes in the userscript.
 API_TOKEN=<paste output of: openssl rand -hex 32>
 
+# Required — the owner's Discord user ID. Seeds the one Reader every bookmark
+# belongs to; the value is the snowflake in your Discord profile (Settings →
+# Advanced → Developer Mode → right-click your name → Copy User ID).
+OWNER_DISCORD_ID=<discord user id>
+
 # CORS allowlist — leave as-is unless a site changes hostname.
 ALLOWED_ORIGINS=https://asuracomic.net,https://asurascans.com,https://demonicscans.org,https://comix.to,https://kagane.to
 
@@ -250,7 +255,7 @@ it; see `REDEPLOY.md` §1 for when to remove it.)
 | `fetch` fails in the userscript, `curl` works | Origin missing from `ALLOWED_ORIGINS`, or mixed content (backend not HTTPS). |
 | 401 with the right token | Trailing space/newline in `API_TOKEN`; regenerate and restart. |
 | Panel button absent | URL didn't match an adapter, or user scripts disabled in Bromite. |
-| `compose ... config` errors about `API_TOKEN` or `POSTGRES_PASSWORD` | Run compose from the dir with `.env`, or export the vars. Both are required and neither has a fallback. |
+| `compose ... config` errors about `API_TOKEN`, `OWNER_DISCORD_ID` or `POSTGRES_PASSWORD` | Run compose from the dir with `.env`, or export the vars. All three are required and none has a fallback. |
 | `bookmark-api` restarts in a loop, `password authentication failed for user "bookmarks"` | `POSTGRES_PASSWORD` was changed after first boot; Postgres only applies it to an empty `postgres-data`. Restore the old value, or reset the role (`REDEPLOY.md` troubleshooting). |
 | `bookmark-api` never logs `listening on :8080` | It is blocked on `postgres` passing `pg_isready`, or a migration failed. `docker compose -f docker-compose.yml -f docker-compose.prod.yml logs postgres`. |