feat(db): dedicated non-superuser runtime role for RLS

The app connected as 'postgres' — the bootstrap SUPERUSER — which
bypasses row-level security unconditionally (FORCE RLS does not apply
to superusers), making any future policy a silent no-op. This is the
prerequisite commit for the RLS visibility work (plan
~/.opencode/plan/server-rls-library-visibility.md REV 2, §4.0).

What changed:
- database/schema/z_rls_role.sh: creates the bookhoard_srv role
  (LOGIN, non-superuser, non-bypassrls) and transfers ownership of
  every public table/sequence/function + schema + database to it.
  Ownership (not mere grants) because executeSchema re-runs the whole
  schema.sql on EVERY boot and needs owner DDL. Idempotent, catalog-
  only — no user data is read or written.
- docker-compose.yml app service + docker-compose.dev.yml tests
  service: DATABASE_USER postgres -> bookhoard_srv, DATABASE_PASSWORD
  DBPASS -> DBSRVPASS (new .env key, generated locally).
- POSTGRES_USER: postgres stays as the container bootstrap account;
  on FRESH volumes z_rls_role.sh runs automatically via
  docker-entrypoint-initdb.d AFTER schema.sql (alphabetical: z > s).

Verified on the dev stack: role has rolsuper=f, rolbypassrls=f, owns
all 40 tables and every public function; single boot as bookhoard_srv
executed schema.sql cleanly (CREATE EXTENSION IF NOT EXISTS pg_trgm
skips fine as non-superuser when the extension exists); health +
authenticated smoke pass. Behavior otherwise unchanged — no policies
yet.

database/schema/z_rls_role.sh

@@ -0,0 +1,69 @@
+#!/bin/bash
+# RLS role bootstrap — plan §4.0 (~/.opencode/plan/server-rls-library-visibility.md REV 2).
+#
+# Creates the NON-SUPERUSER runtime role `bookhoard_srv` and makes it the
+# OWNER of everything in the public schema, so the app can keep running
+# executeSchema's per-boot DDL while being subject to RLS (FORCE).
+#
+# The bootstrap superuser (POSTGRES_USER, "postgres") is UNTOUCHED and
+# remains the cluster/database bootstrap account; the app never connects
+# as it.
+#
+# Runs AUTOMATICALLY on fresh volumes: docker-entrypoint-initdb.d executes
+# initdb.d entries in alphabetical order — "z_rls_role.sh" sorts AFTER
+# "schema.sql" (digits/uppercase sort before lowercase 's'), so the tables
+# exist before ownership transfers. It runs as the postgres superuser with
+# the db container's env (DBSRVPASS comes from .env via compose env_file).
+#
+# EXISTING volumes (initdb.d never re-runs): apply manually ONCE, as the
+# superuser, BEFORE switching the app's DATABASE_USER:
+#   docker exec -i bookhoard_db bash -s < database/schema/z_rls_role.sh
+#
+# IDEMPOTENT: safe to re-run (role creation is guarded; ownership ALTERs
+# and the password set are idempotent).
+#
+# DATA SAFETY: every statement here is catalog-only. No user row is read,
+# rewritten, or deleted — reading progress, annotations, highlights,
+# bookmarks, collections, and media rows are untouched.
+set -euo pipefail
+: "${DBSRVPASS:?DBSRVPASS must be set in the db container env (.env via env_file)}"
+: "${POSTGRES_USER:?POSTGRES_USER must be set (the image sets it)}"
+: "${POSTGRES_DB:?POSTGRES_DB must be set (the image sets it)}"
+
+psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" --dbname "$POSTGRES_DB" \
+     -v role_pass="$DBSRVPASS" <<'EOSQL'
+\echo '== RLS role bootstrap: creating/refreshing bookhoard_srv =='
+
+-- 1. The runtime role: LOGIN, NO superuser, NO bypassrls, NO createrole.
+DO $$
+BEGIN
+  IF NOT EXISTS (SELECT 1 FROM pg_roles WHERE rolname = 'bookhoard_srv') THEN
+    CREATE ROLE bookhoard_srv LOGIN;
+  END IF;
+END $$;
+ALTER ROLE bookhoard_srv LOGIN PASSWORD :'role_pass';
+
+-- 2. Ownership transfer — the runtime role must own the objects so
+--    executeSchema (schema.sql re-runs on EVERY app boot) keeps working,
+--    and so FORCE ROW LEVEL SECURITY can make the owner subject to the
+--    policies. Catalog-only: flips ownership pointers, touches no rows.
+DO $$
+DECLARE r RECORD;
+BEGIN
+  FOR r IN SELECT tablename FROM pg_tables WHERE schemaname = 'public' LOOP
+    EXECUTE format('ALTER TABLE public.%I OWNER TO bookhoard_srv', r.tablename);
+  END LOOP;
+  FOR r IN SELECT sequencename FROM pg_sequences WHERE schemaname = 'public' LOOP
+    EXECUTE format('ALTER SEQUENCE public.%I OWNER TO bookhoard_srv', r.sequencename);
+  END LOOP;
+  FOR r IN SELECT p.oid::regprocedure AS fn FROM pg_proc p
+           JOIN pg_namespace n ON p.pronamespace = n.oid
+           WHERE n.nspname = 'public' AND p.prokind = 'f' LOOP
+    EXECUTE format('ALTER FUNCTION %s OWNER TO bookhoard_srv', r.fn);
+  END LOOP;
+END $$;
+ALTER SCHEMA public OWNER TO bookhoard_srv;
+ALTER DATABASE bookhoard OWNER TO bookhoard_srv;
+
+\echo '== done: bookhoard_srv owns the public schema =='
+EOSQL