feat(upload): avatar and calendar cover via local disk storage.

POST /v1/user/me/avatar and /v1/calendars/:id/cover; GET /v1/media;
UPLOAD_DIR on eventhub-data volume; MIME/size limits.
Refs EventHub/EventHubBack#71
Refs EventHub/EventHubSpec#8

docker/.env.example

@@ -30,4 +30,7 @@ SMTP_TLS=if_available
 PUBLIC_APP_URL=https://stage.calentiq.com
 # Окно напоминаний о booking (часы до старта; Back#70)
 REMINDER_LEAD_HOURS=24
+# Uploads (avatar/cover) — каталог на volume /app/data (Back#71)
+UPLOAD_DIR=/app/data/uploads
+UPLOAD_MAX_BYTES=2097152