Add PocketBase schema migrations and migration pipeline

- 8 collections: vereine, gruppen, mitglieder, beitraege, einzuege,
  termine, nachrichten, push_subscriptions
- verein_id relation added to users (multi-tenant isolation)
- API rules enforce tenant separation via @request.auth.verein_id
- docker-compose: --migrationsDir=/pb_data/migrations flag + volume mount
- Makefile: migrations sync step added to deploy target

pocketbase/pb_migrations/1779215883_updated_users.js

@@ -0,0 +1,29 @@
+/// <reference path="../pb_data/types.d.ts" />
+migrate((app) => {
+  const collection = app.findCollectionByNameOrId("_pb_users_auth_")
+
+  // add field
+  collection.fields.addAt(10, new Field({
+    "cascadeDelete": false,
+    "collectionId": "pbc_3589557411",
+    "help": "",
+    "hidden": false,
+    "id": "relation145676011",
+    "maxSelect": 1,
+    "minSelect": 0,
+    "name": "verein_id",
+    "presentable": false,
+    "required": false,
+    "system": false,
+    "type": "relation"
+  }))
+
+  return app.save(collection)
+}, (app) => {
+  const collection = app.findCollectionByNameOrId("_pb_users_auth_")
+
+  // remove field
+  collection.fields.removeById("relation145676011")
+
+  return app.save(collection)
+})