Add self-hosted PocketBase stack with auto-migrations

- docker-compose.yml: app + pocketbase services (PB internal to the network)
- pocketbase/: pinned PocketBase image, superuser bootstrap via env vars
- pb_migrations: production schema (collections, rules, public_users view)
  applied automatically on first boot
- pocketbase/pb_schema.json: schema export kept as reference
- README: full-stack deployment docs

pocketbase/entrypoint.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -e
+
+# Optionally create/update the admin (superuser) account on boot.
+# Set PB_SUPERUSER_EMAIL and PB_SUPERUSER_PASSWORD as env vars to enable.
+# The password must be at least 8 characters.
+if [ -n "$PB_SUPERUSER_EMAIL" ] && [ -n "$PB_SUPERUSER_PASSWORD" ]; then
+    ./pocketbase superuser upsert "$PB_SUPERUSER_EMAIL" "$PB_SUPERUSER_PASSWORD" || true
+fi
+
+# `serve` automatically applies pending migrations from ./pb_migrations
+exec ./pocketbase serve --http=0.0.0.0:8090